x/tools/gopls: 'run file benchmarks' codelens for files without benchmark functions

[hyangah]

When enabled the test codelens ("gopls": { "codelenses": { "test": true } }), gopls was sending run file benchmarks codelens for files that don't contain any benchmarks.
Clicking the codelens leads to failure.

[gopherbot]

Change https://golang.org/cl/278780 mentions this issue: internal/lsp/source: only show "run file benchmarks" if available