[javadoc] Fix incorrect exception type in Javadoc example (#1563)

Co-authored-by: gaf <gide1011@h-ka.de>

Author: GAFMONKEY

src/main/java/org/apache/commons/lang3/CharUtils.java

@@ -270,7 +270,7 @@ public static boolean isOctal(final char ch) {
      * <pre>
      *   CharUtils.toChar(' ')  = ' '
      *   CharUtils.toChar('A')  = 'A'
-     *   CharUtils.toChar(null) throws IllegalArgumentException
+     *   CharUtils.toChar(null) throws NullPointerException
      * </pre>
      *
      * @param ch  the character to convert
@@ -305,7 +305,7 @@ public static char toChar(final Character ch, final char defaultValue) {
      * <pre>
      *   CharUtils.toChar("A")  = 'A'
      *   CharUtils.toChar("BA") = 'B'
-     *   CharUtils.toChar(null) throws IllegalArgumentException
+     *   CharUtils.toChar(null) throws NullPointerException
      *   CharUtils.toChar("")   throws IllegalArgumentException
      * </pre>
      *
@@ -420,7 +420,7 @@ public static int toIntValue(final char ch, final int defaultValue) {
      *
      * <pre>
      *   CharUtils.toIntValue('3')  = 3
-     *   CharUtils.toIntValue(null) throws IllegalArgumentException
+     *   CharUtils.toIntValue(null) throws NullPointerException
      *   CharUtils.toIntValue('A')  throws IllegalArgumentException
      * </pre>
      *