#792 #814 Create command to remove NOT NULL constraints

- Create command under /authme debug that allows to change the 'nullable' status of MySQL columns (currently last date and email only)
   - We need to offer a default value for forum integrations that have a NOT NULL email column. Offering a command avoids us from force-migrating existing databases while still offering migrations in both directions
- Change in default value handling: lack of values are not handled by setting default values to the PlayerAuth anymore, and reading a default value from the database into a PlayerAuth will be translated into null by the PlayerAuth builder
- When a new database is created, email and lastlogin are now nullable and lack a default a value

Open points:
- Finish MySqlDefaultChangerTest
- Revise purging logic (#792)
- Allow to have more columns nullable (#814)

src/test/java/fr/xephi/authme/api/v3/AuthMeApiTest.java

@@ -162,20 +162,36 @@ public class AuthMeApiTest {
     public void shouldGetLastLogin() {
         // given
         String name = "David";
-        Player player = mockPlayerWithName(name);
         PlayerAuth auth = PlayerAuth.builder().name(name)
-            .lastLogin(1501597979)
+            .lastLogin(1501597979L)
             .build();
         given(playerCache.getAuth(name)).willReturn(auth);
 
         // when
-        Date result = api.getLastLogin(player.getName());
+        Date result = api.getLastLogin(name);
 
         // then
         assertThat(result, not(nullValue()));
         assertThat(result, equalTo(new Date(1501597979)));
     }
 
+    @Test
+    public void shouldHandleNullLastLogin() {
+        // given
+        String name = "John";
+        PlayerAuth auth = PlayerAuth.builder().name(name)
+            .lastLogin(null)
+            .build();
+        given(dataSource.getAuth(name)).willReturn(auth);
+
+        // when
+        Date result = api.getLastLogin(name);
+
+        // then
+        assertThat(result, nullValue());
+        verify(dataSource).getAuth(name);
+    }
+
     @Test
     public void shouldReturnNullForUnavailablePlayer() {
         // given