Remove empty tag set from info page

[LotharSee]

Similar to #328 but for the info page.

Currently we report things like:

  --- Receiver stats (1 min) ---

  -> tags: go, 1.9.1, gc-amd64-linux, v0.5.0

    Traces received: 30425 (19674323 bytes)
    Spans received: 60850
    Services received: 0 (0 bytes)
    Total data received: 19674323 bytes

  ------------------------------

  --- Receiver stats (1 min) ---

  -> tags: None

    Traces received: 0 (0 bytes)
    Spans received: 0
    Services received: 0 (0 bytes)
    Total data received: 0 bytes

  ------------------------------

This PR removes the latter section.