Make type not required if the template already provides one (#8257)

Such as with missile units

core/src/mindustry/mod/ContentParser.java

@@ -461,12 +461,13 @@ public class ContentParser{
                 }
 
                 var typeVal = value.get("type");
-
+                if(unit.constructor == null || typeVal != null){
                     if(typeVal != null && !typeVal.isString()){
                         throw new RuntimeException("Unit '" + name + "' has an incorrect type. Types must be strings.");
                     }
 
                     unit.constructor = unitType(typeVal);
+                }
             }else{
                 unit = locate(ContentType.unit, name);
             }