use nil as default return value for MapCount

[neonstalwart]

this fixes #2271 and includes #2290

if someone could direct me where to put a test i'd be glad to try to add one

[otoolep]

@neonstalwart -- thanks. You should add a unit test to here:

https://github.com/influxdb/influxdb/blob/master/cmd/influxd/server_integration_test.go#L888

feel free to reset the database and write a little bit of data, then query it. Let me know if you have any questions.

[otoolep]

Once a unit test is added (I can add it if necessary), I test it by hand too, then we can merge this.

[neonstalwart]

ok, i'll take a shot at adding the unit test - i think it looks like something i can manage. if you get impatient, go ahead with it yourself - it won't hurt my feelings

[otoolep]

Re-opened at the PR below, so I can debug the test failure.

#2295