[clangd] Return earlier when snippet is empty

Fixes github.com/clangd/clangd/issues/1216

If the Snippet string is empty, Snippet.front() would trigger a crash.
Move the Snippet->empty() check up a few lines to avoid this. Should not
break any existing behavior.

Differential Revision: https://reviews.llvm.org/D134137

(cherry picked from commit 60528c69)