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

.internal/createCaseFirst.js

@@ -11,6 +11,10 @@ import stringToArray from './stringToArray.js'
  */
 function createCaseFirst(methodName) {
   return (string) => {
+    if (!string) {
+      return ''
+    }
+
     const strSymbols = hasUnicode(string)
       ? stringToArray(string)
       : undefined