Exception for spelling suggestions thrown while editing

[DanielRosenwasser]

It's a bit difficult to repro this, but here's the stack trace

TypeError: Cannot read property 'length' of undefined
    at Object.unescapeLeadingUnderscores (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:8552:18)
    at Object.getSuggestionForNonexistentProperty (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:19514:84)
    at Object.getActionsForCorrectSpelling [as getCodeActions] (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:72770:38)
    at C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:72526:33
    at Object.forEach (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:1291:30)
    at Object.getFixes (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:72525:16)
    at C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:75371:35
    at Object.flatMap (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:1524:25)
    at Object.getCodeFixesAtPosition (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:75369:23)
    at IOSession.Session.getCodeFixes (C:\Users\drosen\AppData\Roaming\npm\node_modules\typescript\lib\tsserver.js:81264:64)

[weswigham]

getSuggestionForNonexistentProperty can return undefined (when there's no suggestion), but unescapeLeadingUnderscores and escapeLeadingUnderscores (previously unescapeIdentifier and escapeIdentifier) are not undefined-propagating functions. The services layer will handle the undefined return just fine, so I've opened #17465 to make them undefined-propagating.

If you have a repro (I think something where getSuggestionForNonexistentProperty is called as part of a codefix and has no results, if it narrows your difficult-to-repro conditions), I can add it as a test.