Add check for existing coverage files to merge_coverage function (mod…

[patrick-studer]

If simulation is run with coverage enabled but no testcases were executed (because filtering), the merge_coverage procedure does end in an error. This pre-check checks, if there is anything to merge before performing the merge command.

Resolves #1208.

Result (with ModelSim):

$ make sim
WARNING - Found no test benches using current filter rule:

    Filters entities and modules that have a runner_cfg generic/parameter

    Gives warning when design_unit matches tb_* or *_tb without having a runner_cfg
    Gives warning when a design_unit has  a runner_cfg but the name does not match tb_* or *_tb

Re-compile not needed

No tests were run!
WARNING - No existing coverage files found. Skipping coverage database creation.