fix: prevent crash when concatenating null strings with + operator

[cs01]

Summary

• "val: " + nullString now correctly produces "val: null" instead of crashing (segfault from strlen(null))
• Null literal and undefined literal expressions produce "null" and "undefined" strings directly
• Nullable string variables are null-checked at runtime with a select instruction

Test plan

• New fixture strings/string-concat-null.ts tests null, non-null, and normal string concatenation
• npm run verify:quick passes (all tests + Stage 1 self-hosting)

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