[lldb] Respect empty arguments in target.run-args

Currently empty arguments are not respected. They are silently dropped
in two places: (1) when extracting them from the target.run-args
setting and (2) when constructing the lldb-argdumper invocation.

(1) is actually a regression from a few years ago. We did not always
drop empty arguments. See 31d97a5c.

rdar://106279228

Differential Revision: https://reviews.llvm.org/D145450