The property "migrateToProcessDefinitionVersion" is optional, if it i…

…s not set it takes the latest process definition (see AbstractDynamicStateManager#resolveProcessDefinition()) (#3714)

modules/flowable-engine/src/main/java/org/flowable/engine/impl/migration/ProcessInstanceMigrationDocumentBuilderImpl.java

@@ -129,7 +129,7 @@ public ProcessInstanceMigrationDocument build() {
             if (migrateToProcessDefinitionKey == null) {
                 throw new FlowableException("Process definition key cannot be null");
             }
-            if (migrateToProcessDefinitionVersion == null || migrateToProcessDefinitionVersion < 0) {
+            if (migrateToProcessDefinitionVersion != null && migrateToProcessDefinitionVersion < 0) {
                 throw new FlowableException("Process definition version must be a positive number");
             }
         }