Guard transpileComments against undefined trivia tokens

[TwitchBronBron]

Summary

Fixes a crash during transpile when an @annotation has no leading trivia (whitespace, newline, or comment) before its @ token — typical for annotations that appear at column 0 of a file.

Repro

@annotation
sub main()
    print "main"
end sub

TypeError: Cannot read properties of undefined (reading 'filter')
    at BrsTranspileState.transpileComments (src/parser/TranspileState.ts:144)
    at AnnotationExpression.transpile (src/parser/Expression.ts:2266)
    at BrsTranspileState.transpileAnnotations (src/parser/BrsTranspileState.ts:54)
    at FunctionStatement.transpile (src/parser/Statement.ts:631)
    ...
    at BscPlugin.afterSerializeFile (src/bscPlugin/BscPlugin.ts:119)

Root cause

Lexer.scanToken (src/lexer/Lexer.ts:1082) initializes every token's leadingTrivia to undefined and only assigns an array when trivia accumulated before the token. So a token with no preceding whitespace/newlines/comments keeps leadingTrivia === undefined.

AnnotationExpression.leadingTrivia returns this.tokens.at?.leadingTrivia — undefined in that case — and AnnotationExpression.transpile forwards it directly to state.transpileComments, which calls .filter on it without a guard.

Fix

Add a nullish guard at the top of transpileComments, matching the existing guard in transpileLeadingComments (same file, line 130) and the nullish-tolerant trivia helpers in util.ts (hasLeadingComments, getLeadingComments).

Test plan

• New failing test in BrsFile.spec.ts (does not crash when annotation has undefined leadingTrivia) reproduces the crash with the exact stack trace above on v1 HEAD.
• Test passes after the fix.
• Full suite green: 4179 passing, 9 pending, 0 failing.

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