Update strings readme (#350)

README.md

@@ -179,18 +179,14 @@ used as a literal match inside of a RegExp.
 
 `isEmpty` checks if a string is `null` or empty.
 
+`isNotEmpty` checks if a string is not `null` and not empty.
+
 `equalsIgnoreCase` checks if two strings are equal, ignoring case.
 
 `compareIgnoreCase` compares two strings, ignoring case.
 
 `flip` flips the order of characters in a string.
 
-`nullToEmpty` turns `null` to empty string, and returns non-empty strings
-unchanged.
-
-`emptyToNull` turns empty string to `null`, and returns non-empty strings
-unchanged.
-
 `repeat` concatenates a string to itself a given number of times.
 
 `loop` allows you to loop through characters in a string starting and ending at