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

pad.js

@@ -25,7 +25,7 @@ import stringSize from './.internal/stringSize.js'
 function pad(string, length, chars) {
   const strLength = length ? stringSize(string) : 0
   if (!length || strLength >= length) {
-    return string
+    return (string || '')
   }
   const mid = (length - strLength) / 2
   return (