[clangd] Fix find_program() result check when searching for gRPC

`find_program(<VAR> ...)` sets <VAR> to <VAR>-NOTFOUND if nothing was found.
So we need to compare <VAR> with "<VAR>-NOTFOUND" or just use `if([NOT] <VAR>)`, because `if(<VAR>)` is false if `<VAR>` ends in the suffix -NOTFOUND.

Reviewed By: kbobyrev

Differential Revision: https://reviews.llvm.org/D85958