Theme JSON: Check for null values to cater for blockGap

[ramonjd]

Syncs changes from

• Theme JSON: Check for null values to cater for blockGap gutenberg#59258

When resolving theme.json preset variables, add a check to make sure the value is not empty before we run it through strpos() and preg_match_all().

Run the tests:

npm run test:php -- --filter Tests_Theme_wpThemeJson

Trac ticket: https://core.trac.wordpress.org/ticket/60613

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Props ramonopoly, mukesh27, get_dave.

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[mukeshpanchal27]

This ticket was committed in 6.3 why we mention it here?

[ramonjd]

Oh I added it because it wasn't there when the test was originally added.

It was just for the sake of completeness. I guess it's not really related to this PR and can be removed. 👍🏻

[getdave]

The tests passed for me and this matches the original PR.

I left two nits.

[getdave]

@youknowriad I think this one is ready once the nits are addressed.

[getdave]

@youknowriad LGTM

[swissspidy]

https://core.trac.wordpress.org/changeset/57716

[youknowriad]

Commit https://core.trac.wordpress.org/changeset/57716