Fix problem when enum value is null

Correctly sets the member of type enum to null if needed. Avoiding an
exception because Enum.name() is null.

src/main/java/redis/clients/johm/JOhmUtils.java

@@ -249,7 +249,7 @@ public static Object convert(final Class<?> type, final String value) {
             }
 
             if (type.isEnum() || type.equals(Enum.class)) {
-                return Enum.valueOf((Class<Enum>)type, value);
+                return value == null ? null : Enum.valueOf((Class<Enum>)type, value);
             }
 
             // Raw Collections are unsupported

src/test/java/redis/clients/johm/ConvertorTest.java

@@ -222,6 +222,9 @@ public void testEnumConvertors() {
 
         converted = JOhmUtils.Convertor.convert(FooEnum.class, "FOO");
         assertNotSame(FooEnum.BAZZ, converted);
+
+        converted = JOhmUtils.Convertor.convert(FooEnum.class, null);
+        assertEquals(null, converted);
     }
 
     @Test