[ADT] Allow empty string in StringSet

Also add a test case to wasm-ld that asserts without this change.
Internally wasm-ld builds a StringMap of exported functions and it seems
like allowing empty string in the set is preferable to adding checks.

This assert looks like it was most likely just a historical accident.
It started life here purely to support InputLanguagesSet:

  eeac27e3

Then got extracted here:

  e57a4033

Then got moved to AST here

  5c48bae2

With the `InLang` paramater name still intact which suggested is
InputLanguagesSet origins.

Differential Revision: https://reviews.llvm.org/D74589