[clang] [clang-scan-deps] Ignore import/include directives with missing filenames (PR #99520)

[Argyrios Kyrtzidis via cfe-commits]

akyrtzi wrote:

This is similar feedback that I gave for https://github.com/llvm/llvm-project/pull/97654, could you test the change with a unit test in `DependencyDirectivesScannerTest.cpp` instead? 
It's much simpler to setup and execution-wise it's orders of magnitude more lightweight than a whole new lit test.

For reference see the test change in that PR.

https://github.com/llvm/llvm-project/pull/99520