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

padStart.js

@@ -26,7 +26,7 @@ function padStart(string, length, chars) {
   const strLength = length ? stringSize(string) : 0
   return (length && strLength < length)
     ? (createPadding(length - strLength, chars) + string)
-    : string
+    : (string || '')
 }
 
 export default padStart