Tests: Fix ConfigSources test with empty CMAKE_BUILD_TYPE

The test requires a non-empty `CMAKE_BUILD_TYPE` to function on
single-config generators.  Force a non-empty configuration if the test
is configured explicitly with empty `-DCMAKE_BUILD_TYPE=`.
12 jobs for test-ConfigSources-config
Status Name Job ID Coverage
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-check-master

passed ghostflow-stager