Make sure _meta and _links are mandatory output in the PMA for page, components and content

We currently have

@JsonProperty("_links")
@JsonInclude(Include.NON_NULL)
public Map<String, LinkModel> getLinksMap() 

and

@JsonProperty("_meta")
@JsonInclude(Include.NON_NULL)
public Map<String, Object> getMetadataMap() 

Since the SPA SDK wants to assume _links and _meta are always there, it seems we need to remove '@JsonInclude(Include.NON_NULL)' and also add a unit test to confirm that if link map is null or metaDataMap is null, the PMA still contains _links and _meta