Ignore empty width attribute in columns

The editor was crashing when attempting to render a column block which contained an empty width attribute (i.e `<!-- wp:column {"width":""} -->`.
The exception occurred when accessing a property of an undefined object, so adding a default object (`|| {}`) fixes this by allowing the property to be accessed.

packages/components/src/mobile/utils/use-unit-converter-to-mobile.native.js

@@ -37,7 +37,7 @@ const getValueAndUnit = ( value, unit ) => {
 
 const convertUnitToMobile = ( containerSize, globalStyles, value, unit ) => {
 	const { width, height } = containerSize;
-	const { valueToConvert, valueUnit } = getValueAndUnit( value, unit );
+	const { valueToConvert, valueUnit } = getValueAndUnit( value, unit ) || {};
 	const { fontSize = 16 } = globalStyles || {};
 
 	switch ( valueUnit ) {