Check that support is defined in useCustomSides

Previously the hook was only used if the feature was known to be
supported. Now that it is used in checking the spacing configuration,
it is possible for `support` to be undefined here.

packages/block-editor/src/hooks/dimensions.js

@@ -148,7 +148,7 @@ export function useCustomSides( blockName, feature ) {
 	const support = getBlockSupport( blockName, SPACING_SUPPORT_KEY );
 
 	// Skip when setting is boolean as theme isn't setting arbitrary sides.
-	if ( typeof support?.[ feature ] === 'boolean' ) {
+	if ( ! support || typeof support[ feature ] === 'boolean' ) {
 		return;
 	}