fix: correct style issues and bugs in ivy_guc.c

[OreoYang]

Summary

This PR fixes several style issues and bugs in src/backend/utils/misc/ivy_guc.c:

• Fixed typo: Changed "shown as blow" to "shown below" in comment (line 59)
• Fixed duplicate period: Removed extra period in error hint message (line 479)
• Fixed null terminator bug: Corrected placement of null terminator in nls_case_conversion function (lines 537, 545)
  • The null terminator was being set inside the loop, which would overwrite the last character
  • Now correctly placed after the loop completion
• Code style: Removed extra blank lines for consistent formatting (lines 75, 77)
• License header: Added missing period in Apache License header (line 6)

Test Plan

• Build completed successfully
• All Oracle compatibility tests passed (make oracle-check)
• No regression test failures

Files Changed

• src/backend/utils/misc/ivy_guc.c: 1 file changed, 14 insertions(+), 17 deletions(-)

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