chore: default sep to empty string

Author: akulsr0

lib/util/propTypesSort.js

@@ -185,7 +185,7 @@ function fixPropTypesSort(
       );
 
       const sourceCodeText = getText(context);
-      let separator = null;
+      let separator = '';
       source = nodes.reduceRight((acc, attr, index) => {
         const sortedAttr = sortedAttributes[index];
         const commentNode = commentnodeMap.get(sortedAttr);