Treat null/nil server capabilities as false

Treat null/nil server capabilities as false

Some language servers may specify null for some capabilities in the list
of server capabilities. This does not conform to the specification, but
treating it as false is more reasonable than treating it as true.

A current example is the PHP language server. which specifies null for
every capability it does not handle, like documentHighlightProvider.
This would cause Eglot to send constant textDocument/documentHighlight
requests, which all timed out.

* eglot.el (eglot--server-capable): Change the handling of null values
  for capabilities to treat them as false instead of true.

Copyright-paperwork-exempt: yes