code: public static String stripToNull(String str) {
          if (str == null) {
              return null;
          }
          str = strip(str, null);
          return str.isEmpty() ? 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 CharUtils#isWhitespace(char)}.</p>
  
  <pre>
  StringUtils.stripToNull(null)     = null
  StringUtils.stripToNull("")       = null
  StringUtils.stripToNull("   ")    = null
  StringUtils.stripToNull("abc")    = "abc"
  StringUtils.stripToNull("  abc")  = "abc"
  StringUtils.stripToNull("abc  ")  = "abc"
  StringUtils.stripToNull(" abc ")  = "abc"
  StringUtils.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