The class `lsp.Client` has a public member `server_capabilities`,
which is assumed to be non-nil once initialized, as documented in
`:help vim.lsp.client`. Due to the possibility that it may be nil
before initialization, `lsp.Client` was not having a proper lua type
annotations on the field `server_capabilities`.
Instead of having a nil `server_capabilities` until initialized in
the RPC response callback, we can have an initial value of empty table.
This CHANGES the behavior of the `server_capabilities` field in a way
that it is no longer `nil` until initialization. Note that, as
already documented, `server_capabilities` should never be nil when
it is once initialized and thus ready to be used in user configs.
0e7e25af20
