Fix error on completion 'null' value

[evidolob]

What does this PR do?

Fix error during completion when schema contains null value in enum, example:

{
   "type": "object",
   "properties": {
      "kind": {
        "enum": [
          "project",
          null
       ],
     }
   }
}

With this PR error is gone, but I have doubt about behaviour.
We generate completion item with label null but insert only new line\n.
According to yaml spec

kind:

and

kind: null

are equal, but maybe it would be better if we will insert null\n instead of just \n?

What issues does this PR fix or reference?

Found during #344 investigation.

Is it tested? How?

Use example schema and yaml content:

kind: 

call completion after kind: , you should see completion list with two items project and null.

[coveralls]

Coverage increased (+0.6%) to 80.22% when pulling 560de41 on evidolob:fix-error-on-null-completion into 33b090a on redhat-developer:master.

[JPinkney]

According to yaml spec

kind:
and

kind: null
are equal, but maybe it would be better if we will insert null\n instead of just \n?

I think I like the idea of inserting null\n a bit better. I tried with it just inserting \n but I'm not sure when anyone would actually use the completionitem for that rather then just pressing enter but I could be wrong

[evidolob]

@JPinkney Done, just note, to alight with other string completion we will insert just null string, without \n