Add null check to prevent errors in get_block_template filter

[okmttdhr]

What?

This PR adds null check to prevent Error : Attempt to assign property "content" on null in gutenberg_replace_pattern_blocks_get_block_template filter introduced in #60349.

Why?

Since the first param $template is WP_Block_Template or null if there isn't one.
https://github.com/WordPress/wordpress-develop/blob/8d0aed455b1790c5d51386f7675d9e8d68e48edf/src/wp-includes/block-template-utils.php#L1113

How?

Added null check before assigning the content.

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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Co-authored-by: okmttdhr <okat@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

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[Mamaduka]

Makes sense.

[ellatrix]

To me that sounds like there's a bug in the get_block_template filter, which should be called at all for null.

[jeremyfelt]

Related, this fixes a fatal error documented in #60638

[colorful-tones]

I've added the Needs PHP backport label, but I'm unsure if it applies here. I'm mostly inferring this from the referenced #60349 which has the Needs PHP backport label. Feel free to remove if it is not applicable.