Add assertion for null class loggers

Null class loggers are created using a incorrect method.
However, AB3 currently do not use null class loggers.

Let's replace the null class check with assertion instead
to follow YAGNI principle.

src/main/java/seedu/address/commons/core/LogsCenter.java

@@ -53,9 +53,7 @@ public static Logger getLogger(String name) {
      * Creates a Logger for the given class name.
      */
     public static <T> Logger getLogger(Class<T> clazz) {
-        if (clazz == null) {
-            return Logger.getLogger("");
-        }
+        assert clazz != null;
         return getLogger(clazz.getSimpleName());
     }