Fixed NPE in GET to /users/:userId

API was tripping when an invalid userId is passed. Fixed it, expect a 404 status if userId is invalid.

Failed to include a file in previous commit.

Closes #51

src/main/java/com/myhome/controllers/mapper/HouseApiMapper.java

@@ -0,0 +1,15 @@
+package com.myhome.controllers.mapper;
+
+import com.myhome.controllers.response.GetHouseDetailsResponse;
+import com.myhome.domain.CommunityHouse;
+import java.util.Set;
+import org.mapstruct.Mapper;
+
+@Mapper
+public interface HouseApiMapper {
+  Set<GetHouseDetailsResponse.CommunityHouse> communityHouseSetToRestApiResponseCommunityHouseSet(
+      Set<CommunityHouse> communityHouse);
+
+  GetHouseDetailsResponse.CommunityHouse communityHouseToRestApiResponseCommunityHouse(
+      CommunityHouse communityHouse);
+}