Fix AttributeError when executing empty or comment-only scripts

[veeceey]

Summary

• Fixes AttributeError: 'NoneType' object has no attribute 'decode' when executing empty or comment-only scripts
• Adds null check for result_status_msg before attempting to decode
• Includes test cases for empty scripts, comment-only scripts, and whitespace-only scripts

Issue

Fixes #1259

When executing a PostgreSQL script that contains only comments (e.g., -- hello) or is empty, PostgreSQL returns no result status message. The code attempted to call .decode() on None, causing an AttributeError.

Changes

• Modified asyncpg/protocol/protocol.pyx to check if result_status_msg is not None before decoding
• Added comprehensive test cases in tests/test_execute.py to verify the fix

Test plan

• Added test case for comment-only script ('-- hello')
• Added test case for whitespace-only script (' ')
• Added test case for empty script ('')
• All tests verify that the execute() call returns None instead of raising an error

This is a natural behavior where executing a script with only comments should be a no-op, which is useful when commenting out migration script contents during development.

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.