[LANG-1804] Fix CharSet#getInstance returns null instead of EMPTY when input setStrs is null (#1530)

* Fix CharSet.getInstance(final String... setStrs) returns null instead of EMPTY when input setStrs is null

* Remove unused import assertNull from CharSetTest

Removed unused import statement for assertNull.

---------

Co-authored-by: Gary Gregory <[email protected]>

Author: IcoreE

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

@@ -151,7 +151,7 @@ public class CharSet implements Serializable {
      */
     public static CharSet getInstance(final String... setStrs) {
         if (setStrs == null) {
-            return null;
+            return EMPTY;
         }
         if (setStrs.length == 1) {
             final CharSet common = COMMON.get(setStrs[0]);

src/test/java/org/apache/commons/lang3/CharSetTest.java

@@ -21,7 +21,6 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotEquals;
-import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -390,6 +389,8 @@ void testEquals_Object() {
     @Test
     void testGetInstance() {
         assertSame(CharSet.EMPTY, CharSet.getInstance((String) null));
+        assertSame(CharSet.EMPTY, CharSet.getInstance((String[]) null));
+        assertSame(CharSet.EMPTY, CharSet.getInstance(null));
         assertSame(CharSet.EMPTY, CharSet.getInstance(""));
         assertSame(CharSet.ASCII_ALPHA, CharSet.getInstance("a-zA-Z"));
         assertSame(CharSet.ASCII_ALPHA, CharSet.getInstance("A-Za-z"));
@@ -400,7 +401,7 @@ void testGetInstance() {
 
     @Test
     void testGetInstance_Stringarray() {
-        assertNull(CharSet.getInstance((String[]) null));
+        assertEquals("[]", CharSet.getInstance((String[]) null).toString());
         assertEquals("[]", CharSet.getInstance().toString());
         assertEquals("[]", CharSet.getInstance(new String[] {null}).toString());
         assertEquals("[a-e]", CharSet.getInstance("a-e").toString());