Make sure headers are not empty in the requires header check

[ernilambar]

When there is Requires at least is plugin header but not in readme, this check should not have been triggered. It generated message like this.

Mismatched Requires at least:  != 4.9. "Requires at least" needs to be exactly the same with that in your main plugin file's header.

So I believe it makes sense to check for not empty for header value before actually checking the comparison.

[github-actions]

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Co-authored-by: ernilambar <[email protected]>
Co-authored-by: davidperezgar <[email protected]>
Co-authored-by: swissspidy <[email protected]>

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[swissspidy]

Is there a test or at least bug description that explains whag this solves?

[ernilambar]

Description updated.