Small error in otfm-gutenberg-spoiler.php

Hi. ogs_admin_inline_style() can return null, which will then be passed to wp_add_inline_style(). This is not good, because then null gets into the stripos function and trigger message:

PHP Deprecated:  stripos(): Passing null to parameter #1 ($haystack) of type string is deprecated in \wp-includes\functions.wp-styles.php on line 90

It also prints an empty style tag in the pages html. To avoid this, you need to add a check in two plugin functions. Here is the corrected part of plugin code:

function ogs_add_admin_inline_style() {
	$style = ogs_admin_inline_style();

	if (!is_null($style)) {
		wp_add_inline_style( 'ogs_style', $style ); // admin inline
	}
}

// add style inline frontend
add_action( 'wp_head', 'ogs_add_frontend_inline_style' );
function ogs_add_frontend_inline_style() {
	$style = ogs_admin_inline_style();

	if (!is_null($style)) {
	echo "<style>" . $style . "</style>\r\n";
	}
}