code: public static String toLowerCase(final String str, final Locale locale) {
          if (N.isNullOrEmpty(str)) {
              return str;
          }
  
          return str.toLowerCase(locale);
      }
docs: <p>
  Converts a String to lower case as per {@link String#toLowerCase(Locale)}
  .
  </p>
  
  <p>
  A {@code null} input String returns {@code null}.
  </p>
  
  <pre>
  StrUtil.toLowerCase(null, Locale.ENGLISH)  = null
  StrUtil.toLowerCase("", Locale.ENGLISH)    = ""
  StrUtil.toLowerCase("aBc", Locale.ENGLISH) = "abc"
  </pre>
  
  @param str
  the String to lower case, may be null
  @param locale
  the locale that defines the case transformation rules, must
  not be null
  @return the lower cased String, {@code null} if null String input
  @since 2.5