Allow creating a label selector object with a key mapped to no value …

[jwforres]

…to handle the exists operator for now

[jwforres]

@spadgett this is to let us create a label selector from the UI code so that clicking on the key part of a label will add the key exists filter

[spadgett]

So you pass JSON like this as the selector?

{
  "mykey": ""
}

Might be good to update the comment above the function that describes the param.

Just the one nit, code LGTM.

[jwforres]

Actually I'm passing

{
  "mykey": null
}

Empty string actually seems to be a valid label value, so i need to update to check explicitly for null/undefined.