handle missing case properly

[sivukhin]

I ran govanish linter and it found one suspicious place:

2025/04/20 15:32:09 module path: /home/sivukhin/code/openai-go
2025/04/20 15:32:09 ready to compile project at path '/home/sivukhin/code/openai-go' for assembly inspection
2025/04/20 15:32:09 ready to parse assembly output
2025/04/20 15:32:10 ready to normalize assembly lines (size 76)
2025/04/20 15:32:10 built func registry: 626 entries
2025/04/20 15:32:10 ready to analyze module AST
2025/04/20 15:32:10 it seems like your code vanished from compiled binary: func=[toRespField], file=[/home/sivukhin/code/openai-go/internal/apijson/subfield.go], lines=[61-61], snippet:
	return resp.Field{}

Indeed, as IsNull() combine both explicit null and missing case, branch IsMissing() were redundant in the previous order of conditions - so compiler removed it from the code.

This looks like a bug - one possible fix is to properly treat Missing case by changing the order of conditions. But maybe there is a better (and safer - can this fix break compatibility?) fix. So feel free to close this PR.