Fix warnings when `filter_the_content` cannot find a valid attachment id

[mikelittle]

This code in inc/class-tachyon.php. Can cause warnings.

$src_per_wp = wp_get_attachment_image_src( $attachment_id, $size );

if ( self::validate_image_url( $src_per_wp[0] ) ) {
	$src = $src_per_wp[0];
	$fullsize_url = true;
...

If the return from wp_get_attachment_image_src is false, which it can be, the code should not continue as if it got the array of image data.

The call to validate_image_url generates a warning:
"Trying to access array offset on value of type bool"

and then the first line in validate_image_url generates another warning:
"parse_url(): Passing null parameter #1 ($url) of type string is deprecated"

I don't understand how the $attachement_id might be invalid, or reference an invalid image, but we know this does happen in the wild.

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Acceptance Criteria

• Content with invalid attachment id does not generate these warnings
• Document testing on the ticket

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For Altis Team Use

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[VlasicVojislav]

Hey team! Please add your planning poker estimate with Zenhub @mikelittle @vladislavhmn @wisyhambolu @jerico