code: public static String toUpperCase(final String str) {
          if (N.isNullOrEmpty(str)) {
              return str;
          }
  
          return str.toUpperCase();
      }
docs: <p>
  Converts a String to upper case as per {@link String#toUpperCase()}.
  </p>
  
  <p>
  A {@code null} input String returns {@code null}.
  </p>
  
  <pre>
  N.toUpperCase(null)  = null
  N.toUpperCase("")    = ""
  N.toUpperCase("aBc") = "ABC"
  </pre>
  
  <p>
  <strong>Note:</strong> As described in the documentation for
  {@link String#toUpperCase()}, the result of this method is affected by
  the current locale. For platform-independent case transformations, the
  method {@link #toLowerCase(String, Locale)} should be used with a specific
  locale (e.g. {@link Locale#ENGLISH}).
  </p>
  
  @param str
  the String to upper case, may be null
  @return the upper cased String, {@code null} if null String input