Fix string methods to handle empty values (#4442)

* Enable strings category methods tests

* Ensure escape, pad, padEnd, padStart, trim, trimEnd, trimStart, unescape return an empty string for falsey values

* Coerce value to string using toString in truncate, capitalize and case methods

* Ensure createCaseFirst returns an empty string for falsey values

escape.js

@@ -41,7 +41,7 @@ const reHasUnescapedHtml = RegExp(reUnescapedHtml.source)
 function escape(string) {
   return (string && reHasUnescapedHtml.test(string))
     ? string.replace(reUnescapedHtml, (chr) => htmlEscapes[chr])
-    : string
+    : (string || '')
 }
 
 export default escape