Prometheus /graph UI loading spinner spins forever if result is null

[zemek]

Using prometheus 2.0, if you go to /graph and graph nonexistant_metric == 0 or absent(up) the UI spinner keeps spinning forever and it looks like the query never finishes

However, if you run the query manually using curl, you do get back
{"status":"success","data":{"resultType":"vector","result":null}}

If you just graph nonexistant_metric == 0 you get back
{"status":"success","data":{"resultType":"vector","result":[]}}
and the query returns immediately through the UI

It seems that the UI doesn't handle result: null correctly

[zemek]

dupe of #3515

[lock]

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