#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/data/auth/PlayerAuthTest.java

@@ -0,0 +1,63 @@
+package fr.xephi.authme.data.auth;
+
+import org.junit.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.nullValue;
+import static org.junit.Assert.fail;
+
+/**
+ * Test for {@link PlayerAuth} and its builder.
+ */
+public class PlayerAuthTest {
+
+    @Test
+    public void shouldRemoveDatabaseDefaults() {
+        // given / when
+        PlayerAuth auth = PlayerAuth.builder()
+            .name("Bobby")
+            .lastLogin(0L)
+            .email("your@email.com")
+            .build();
+
+        // then
+        assertThat(auth.getNickname(), equalTo("bobby"));
+        assertThat(auth.getLastLogin(), nullValue());
+        assertThat(auth.getEmail(), nullValue());
+    }
+
+    @Test
+    public void shouldThrowForMissingName() {
+        try {
+            // given / when
+            PlayerAuth.builder()
+                .email("test@example.org")
+                .groupId(3)
+                .build();
+
+            // then
+            fail("Expected exception to be thrown");
+        } catch (NullPointerException e) {
+            // all good
+        }
+    }
+
+    @Test
+    public void shouldCreatePlayerAuthWithNullValues() {
+        // given / when
+        PlayerAuth auth = PlayerAuth.builder()
+            .name("Charlie")
+            .email(null)
+            .lastLogin(null)
+            .groupId(19)
+            .locPitch(123.004f)
+            .build();
+
+        // then
+        assertThat(auth.getEmail(), nullValue());
+        assertThat(auth.getLastLogin(), nullValue());
+        assertThat(auth.getGroupId(), equalTo(19));
+        assertThat(auth.getPitch(), equalTo(123.004f));
+    }
+}