Eglot: careful when invoking code actions on no symbol at all

Eglot: careful when invoking code actions on no symbol at all

Invoking code actions without a marked region or over a symbol
will trip certain servers up since BEG and END in eglot-code-actions
will be nil, causing 'eglot--pos-to-lsp-position' to assume point (which
is OK) but the 'flymake-diagnostics' call to return all diagnostics.

This causes an absolutely undecipherable JavaScript backtrace to be
sent back to Eglot from typescript-language-server.

Github-reference: https://github.com/joaotavora/eglot/issues/847

* lisp/progmodes/eglot.el (eglot--code-action-bounds): Avoid returning
  (list nil nil)

(cherry picked from commit b5de9ae8010684a5ed0c6f2703077a61d325ccad)