Extended KeywordModel with metadata field fails when field is empty

[tfinez]

When we have a component that has linked a keyword, and this keyword has a metadata field empty, we get an error when loading the Entity Model in the line 144 in this file: https://github.com/sdl/dxa-web-application-java/blob/release/2.0/dxa-framework/dxa-common-api/src/main/java/com/sdl/webapp/common/impl/mapping/SemanticMapperImpl.java

We have extended the KeywordModel, and if the meta has value, there's no problem, but it fails when the field is empty.

More info: https://tridion.stackexchange.com/questions/19089/extended-keywordmodel-with-metadata-fails-when-metadata-empty

[rpannekoek]

Internal Issue ID (for tracking purposes): CRQ-12810