Exit collect_coverage.sh early if LCOV_MERGER is not set.

[c-mita]

Rules that provide InstrumentedFilesInfo but do not define the
_lcov_merger attribute will result in this script being called without
the required LCOV_MERGER environment variable being set.

Since all rules now provide an InstrumentedFilesInfo provider by
default, but are not required to provide the attribute, this scenario
will be encountered far more frequently and should not be an error.

Because of the way coverage collection within Google differs to Bazel,
it isn't safe to change TestActionBuilder to simply not prepare the
collect_coverage.sh script the way it would if no coverage was to be
collected, so we have to change the script itself.

Fixes #13978

[lberki]

Remind me again, why isn't it feasible to add the :lcov_merger attribute to all Starlark test rules?

[c-mita]

Remind me again, why isn't it feasible to add the :lcov_merger attribute to all Starlark test rules?

1. It's a breaking change, since rules that already define _lcov_merger will break.
2. It breaks things internally; rules within Google work a little differently w.r.t. setting up the lcov_merger tool (which is why the original PR, Provide the default lcov_merger to all Starlark tests #13983, wasn't viable). It's typical for Google's test rules to leave it undefined and let the different collect_coverage.sh handle things.

Other little minor issues arise, but the second point is the big one.

[lberki]

On second thought, there isn't any downside for this change, so LGTM. It would be nice to have an overarching design for adding coverage collection for any language, but that shouldn't be a blocker for this change.