Consider empty strings in Text.indexOf

[runarorama]

Fixes this bug:

> ##Text.indexOf "" "foo"

⬇️

Encountered exception:
Data.Text.Lazy.breakOn: empty input
CallStack ( from HasCallStack ):
    error

The empty search string now just always returns the 0 index.

[pchiusano]

LGTM. I'd add a one line regression test to transcripts/builtins.md, @stew already has a bunch of examples there that you can follow.

test> Text.tests.indexOf =
   haystack = "01020304" ++ "05060708" ++ "090a0b0c01"
   needle1 = "01"
   needle2 = "02"
   needle3 = "0304"
   needle4 = "05"
   needle5 = "0405"
   needle6 = "0c"
   needle7 = haystack
   needle8 = "lopez"
   checks [
     Text.indexOf needle1 haystack == Some 0,
     Text.indexOf needle2 haystack == Some 2,
     Text.indexOf needle3 haystack == Some 4,
     Text.indexOf needle4 haystack == Some 8,
     Text.indexOf needle5 haystack == Some 6,
     Text.indexOf needle6 haystack == Some 22,
     Text.indexOf needle7 haystack == Some 0,
     Text.indexOf needle8 haystack == None,
   ]