[clang] [Clang][Driver] Skip empty strings in getAArch64MultilibFlags (PR #97827)

[via cfe-commits]

asmok-g wrote:

Hi, is it expected that this causes flags like `-Wno-something-c++20-aggregate-init` to be unrecognizable and emit `-Wunknown-warning-option` ?

>From my understanding, this is not a desired behavior ? This is causing failures in google compilation so if my understanding is right please provide a fix forward or revert..

https://github.com/llvm/llvm-project/pull/97827