internal/lsp: allow ExecuteCommandParams to be nil
internal/lsp: allow ExecuteCommandParams to be nil

The ExecuteCommandParams field is optional, so it can be set to nil.

Change-Id: If440257e6b64ce5189b0a4b39a6ed7fc867da5af
Reviewed-on: https://go-review.googlesource.com/c/162402
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools