In commit 7a969fe2 (cmMakefile: Refactor API to better handle empty
config values, 2020-06-30, v3.19.0-rc1~567^2), calls to
`GetGeneratorConfigs` that pass `OnlyMultiConfig` only want to get any
configurations listed if the generator is multi-config.  Fix the
implementation to actually do that.

Fixes: #21316