Improve handling of nullable values in iOS renderer

This pull request improves the usage of nullable values that can be returned by various bits of the iOS renderer.

The initial place this appeared was [this rive-ios issue](#330), where in the provided stack trace, we can see the following:

```
...
  "exception" : {"codes":"0x0000000000000000, 0x0000000000000000","rawCodes":[0,0],"type":"EXC_CRASH","signal":"SIGABRT"},
  "termination" : {"flags":0,"code":6,"namespace":"SIGNAL","indicator":"Abort trap: 6","byProc":"Application","byPid":1045},
  "asi" : {"libsystem_c.dylib":["abort() called"]},
  "exceptionReason" : {"arguments":["-[__NSArrayM insertObject:atIndex:]"],"format_string":"*** %s: object cannot be nil","name":"NSInvalidArgumentException","type":"objc-exception","composed_message":"*** -[__NSArrayM insertObject:atIndex:]: object cannot be nil","class":"NSException"},
  "extMods" : {"caller":{"thread_create":0,"thread_set_state":0,"task_for_pid":0},"system":{"thread_create":0,"thread_set_state":0,"task_for_pid":0},"targeted":{"thread_create":0,"thread_set_state":0,"task_for_pid":0},"warnings":0},
  "lastExceptionBacktrace" : [
{"imageOffset":978358,"symbol":"__exceptionPreprocess","symbolLocation":226,"imageIndex":9},
{"imageOffset":81565,"symbol":"objc_exception_throw","symbolLocation":48,"imageIndex":7},
{"imageOffset":123841,"symbol":"-[__NSArrayM insertObject:atIndex:]","symbolLocation":1762,"imageIndex":9},
{"imageOffset":35776,"symbol":"-[RiveStateMachineInstance stateChanges]","symbolLocation":178,"imageIndex":1},
...
```

Peeking into `-[RiveStateMachineInstance stateChanges]` showed that we were not checking if the nullable value was valid before attempting to insert into an array, thus under certain conditions inserting nil (those conditions still tbd; I've asked the dev for a hopefully reproducible file).

I went ahead and peeked at various other portions of the renderer for nullable usage and added some other checks for array insertion that were similar.

Diffs=
cd8fef48f Improve handling of nullable values in iOS renderer (#7529)

Co-authored-by: David Skuza <[email protected]>

.rive_head

@@ -1 +1 @@
-a22d3ea8c780dbaed55b55bc340f25104b7aa134
+cd8fef48ffd302962e63b8452e07b1cce6aa4af6

Source/Renderer/RiveArtboard.mm

@@ -129,7 +129,11 @@ - (NSArray*)animationNames
     NSMutableArray* animationNames = [NSMutableArray array];
     for (NSUInteger i = 0; i < [self animationCount]; i++)
     {
-        [animationNames addObject:[[self animationFromIndex:i error:nil] name]];
+        RiveLinearAnimationInstance* animation = [self animationFromIndex:i error:nil];
+        if (animation != nil)
+        {
+            [animationNames addObject:[animation name]];
+        }
     }
     return animationNames;
 }
@@ -197,7 +201,11 @@ - (NSArray*)stateMachineNames
     NSMutableArray* stateMachineNames = [NSMutableArray array];
     for (NSUInteger i = 0; i < [self stateMachineCount]; i++)
     {
-        [stateMachineNames addObject:[[self stateMachineFromIndex:i error:nil] name]];
+        RiveStateMachineInstance* stateMachine = [self stateMachineFromIndex:i error:nil];
+        if (stateMachine != nil)
+        {
+            [stateMachineNames addObject:[stateMachine name]];
+        }
     }
     return stateMachineNames;
 }

Source/Renderer/RiveFile.mm

@@ -416,7 +416,11 @@ - (NSArray*)artboardNames
 
     for (NSUInteger i = 0; i < [self artboardCount]; i++)
     {
-        [artboardNames addObject:[[self artboardFromIndex:i error:nil] name]];
+        RiveArtboard* artboard = [self artboardFromIndex:i error:nil];
+        if (artboard != nil)
+        {
+            [artboardNames addObject:[artboard name]];
+        }
     }
     return artboardNames;
 }

Source/Renderer/RiveStateMachineInstance.mm

@@ -247,7 +247,11 @@ - (NSArray*)inputNames
 
     for (NSUInteger i = 0; i < [self inputCount]; i++)
     {
-        [inputNames addObject:[[self inputFromIndex:i error:nil] name]];
+        RiveSMIInput* input = [self inputFromIndex:i error:nil];
+        if (input != nil)
+        {
+            [inputNames addObject:[input name]];
+        }
     }
     return inputNames;
 }
@@ -339,7 +343,11 @@ - (NSArray*)stateChanges
 
     for (NSUInteger i = 0; i < [self stateChangedCount]; i++)
     {
-        [inputNames addObject:[[self stateChangedFromIndex:i error:nil] name]];
+        RiveLayerState* state = [self stateChangedFromIndex:i error:nil];
+        if (state != nil)
+        {
+            [inputNames addObject:[state name]];
+        }
     }
     return inputNames;
 }