Enable plugins during --check

[danpf]

Diagnostics reports provided by lua-language-server --check . currently does not use plugins.

For some complex projects, plugins are required for proper type annotations.

To fix this:

1. require plugin to add the 'startReload' / 'initPlugin' callback to the workspace watchlist
2. in a previous PR, pluginargs was changed to support multiple plugins. Previously it had 1 default type, but now it has 2. This means that the default get returns nil instead of an empty table. To smooth this transition, I set it to {} when it is nil.

[sumneko]

Thank you!