<null> fields yeld NPE on getX operations.

[baranowb]

GHPullRequest@645aa696[base=org.kohsuke.github.GHCommitPointer@7ce026d3,head=org.kohsuke.github.GHCommitPointer@7ce69770,additions=1,merged=false,mergeable=,deletions=1,assignee=,state=open,number=1903,comments=2,labels=,title=XXXXX,milestone=,url=PR_URL,id=122473208]

Will throw NPE on .getMergeable()

[baranowb]

https://developer.github.com/v3/pulls "The value of the mergeable attribute can be true, false, or null. If the value is null, this means that the mergeability hasn't been computed yet, and a background job was started to compute it. Give the job a few moments to complete, and then submit the request again. When the job is complete, the response will include a non-null value for the mergeable attribute."

This should be reflected in field. Worse case scenario with Boolean, so null can be one of potential values or with proper enum YES,NO, NOT_DEFINED( or something similar)

[kohsuke]

Without stack trace, I'm afraid I cannot fix this.