[LLVM][Runtimes] Surround unknown runtime with ""

So that when you do something like:
-DLLVM_ENABLE_RUNTIMES=;compiler-rt;libcxx;libcxxabi;libunwind

You get:
Runtime "" is not a supported runtime.  Supported runtimes are:

Instead of:
Runtime is not a supported runtime.  Supported runtimes are:

Which implies that "Runtime" is the name of the runtime you
tried to build, but in fact it was "".