Correctly handle comments on a LexEntry that doesn't have a lexeme

[rmunn]

One project failed Send/Receive on the line var lexeme = GetLexEntry(comment.EntryRef).Lexeme.FirstNonEmptyString(); in ComvertMongoToLcmComments.cs, with a NullReferenceException. On investigation, the entry was missing a lexeme in Mongo, and so Lexeme.FirstNonEmptyString() was null.FirstNonEmptyString(). The only thing that the lexeme variable is used for in that code is fo human-readable text, so that line should be replaced with:

var lexeme = GetLexEntry(comment.EntryRef).Lexeme;
var lexemeText = (lexeme == null) ? "(no lexeme)" : FirstNonEmptyString();

Or, once we can use modern versions of C# (i.e. once we drop Mono 5):

var lexeme = GetLexEntry(comment.EntryRef)?.Lexeme?.FirstNonEmptyString() ?? "(no lexeme)";

[jasonleenaylor]

I'm pretty sure you can already use that with mono5. There are other new features that you can't.

[megahirt]

This seams like the fix is already thought out here.

Error repro steps:

• [ ] start with working Flex / LF send/receive project
• [ ] delete the data in the lexeme field for the entry (should this even be allowed?)
• [ ] do a S/R and see the error in LFMerge

@josephmyers this seems like a good candidate for testing out the "skip exceptions" feature that we recently developed.

[megahirt]

I wonder if the steps to trigger a "comment exception" could be:

• comment on an entry in LF
• delete the text in the lexeme field
• do a send/receive and see if we get the error (and if we do, then we should fix this issue)

c.f. https://github.com/sillsdev/web-languageforge/issues/1444