lsp: Allow decoded.result to be `false`

Some servers might respond to `workspace/executeCommand` requests with a
boolean result and that could be `false`.

A `false` result should be allowed and not trigger the `on_error`
handler:

    -- Invalid server message
    on_error(client_errors.INVALID_SERVER_MESSAGE, decoded)

Concrete example where this occurred is with eclipse.jdt.ls:

    vim.lsp.buf_request(
      0,
      'workspace/executeCommand',
      {
        command = 'java.project.isTestFile',
        arguments = { vim.uri_from_bufnr(0), },
      },
      function(err, _, resp)
        print(vim.inspect(err), vim.inspect(resp))
      end
    )

0 files changed, 0 insertions, 0 deletions