Modify behavior of `blt_list_append` ?

[kennyweiss]

The Axom project recently fixed a bug related to ENV variables that were not correctly setup -- https://github.com/LLNL/axom/pull/1168

It revealed that blt_list_append generates a FATAL_ERROR when attempting to append empty strings to a list. https://github.com/LLNL/blt/blob/d57f7995ff8299e4612e74ec94e35eacde93cf40/cmake/BLTInstallableMacros.cmake#L40-L42

Is this the desired behavior? Or would it be better if appending an empty string to a list was a no-op?

Ben Liu suggested some additional options:

• blt_list_append should emit a warning in this case since this is often unexpected behavior
• blt_list_append should take an (optional) additional parameter regarding how to treat this case, e.g. FATAL_ERROR, warning or no-op. If we pursue this option, we'd also need to decide on the default behavior.

@white238 noted that CMake's (list(APPEND <empty string>) is a no-op