Person: Remove 'Tags: ' if there are no tags

An additional phrase 'Tags:' is always present even if the user did
not input any tags.

As a result, this is not intuitive as the result message is displaying
unnecessary information.

Let's remove 'Tags:' from the result message if tags are not present.

src/main/java/seedu/address/model/person/Person.java

@@ -111,9 +111,13 @@ public String toString() {
                 .append("; Email: ")
                 .append(getEmail())
                 .append("; Address: ")
-                .append(getAddress())
-                .append("; Tags: ");
-        getTags().forEach(builder::append);
+                .append(getAddress());
+
+        Set<Tag> tags = getTags();
+        if (!tags.isEmpty()) {
+            builder.append("; Tags: ");
+        }
+        tags.forEach(builder::append);
         return builder.toString();
     }