[clang] Fix UB when string.front() is used for the empty string

Compilation database might have empty string as a command line argument.
But ExpandResponseFilesDatabase::expand doesn't expect this and assumes
that string.front() can be used for any argument. It is undefined behaviour if
string is empty. With debug build mode it causes crash in clangd.

Test Plan: check-clang

Differential Revision: https://reviews.llvm.org/D105120