code: public static String stripToNull(String str) {
          str = strip(str, null);
  
          return N.isNullOrEmpty(str) ? null : str;
      }
docs: <p>
  Strips whitespace from the start and end of a String returning
  {@code null} if the String is empty ("") after the strip.
  </p>
  
  <p>
  This is similar to {@link #trimToNull(String)} but removes whitespace.
  Whitespace is defined by {@link Character#isWhitespace(char)}.
  </p>
  
  <pre>
  N.stripToNull(null)     = null
  N.stripToNull("")       = null
  N.stripToNull("   ")    = null
  N.stripToNull("abc")    = "abc"
  N.stripToNull("  abc")  = "abc"
  N.stripToNull("abc  ")  = "abc"
  N.stripToNull(" abc ")  = "abc"
  N.stripToNull(" ab c ") = "ab c"
  </pre>
  
  @param str
  the String to be stripped, may be null
  @return the stripped String, {@code null} if whitespace, empty or null
  String input
  @since 2.0