fix: prevent divide-by-zero in padstart/padend with empty pad string

[cs01]

Summary

• "hello".padStart(10, "") previously caused a divide-by-zero crash in the compiled binary
• Added guard: skip padding when pad string is empty (matches JS behavior — returns original string)
• Same fix applied to both padStart and padEnd

Test plan

• New test fixture string-pad-empty.ts covers padStart/padEnd with empty string, space, and shorter target
• npm run verify:quick passes (tests + stage 1 self-hosting)