Update MuonSelectionTool.cxx

In Run3, summary.cscUnspoiledEtaHits will be left to 0 (default value).
Due to this, getResolutionCategory() may return ResolutionCategory::spoiledCSC, which seems unreasonable (for Run3).

Anyway, this function is currently only used by MuonCalibTool (here),
which only cares for the case of highPt categories (here) which are unaffected by this MR, because of the return statements.

So this MR is expected to change absolutely nothing. It's rather a logical addition.

@Tagging the MCP Calib: @lucam @elrossi