#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/main/java/fr/xephi/authme/api/v3/AuthMeApi.java

@@ -177,7 +177,7 @@ public class AuthMeApi {
         if (auth == null) {
             auth = dataSource.getAuth(playerName);
         }
-        if (auth != null) {
+        if (auth != null && auth.getLastLogin() != null) {
             return new Date(auth.getLastLogin());
         }
         return null;