[clang-tools-extra] allow implicit completion inside empty template argument list (PR #138846)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 requested changes to this pull request.

Looks reasonable to me, but the buildkite run is showing `AllowImplicitCompletions.All` is currently failing

https://github.com/llvm/llvm-project/pull/138846