refactor: allow no source files

Could be used for static viewer without java sources

Author: Sylvain Fraïssé

src/main/java/com/codingame/codemachine/compiler/java/CodinGameJavaCompiler.java

@@ -99,10 +99,8 @@ public static void main(String... args) throws IOException {
                     }
                 }
                 resultCode = success ? 0 : 1;
-            }
-            else {
-                System.err.println("no source file");
-                resultCode = 2;
+            } else {
+                resultCode = 0;
             }
         }