zeta: Various product fixes before Preview release (#23125)

Various fixes for Zeta and one fix that's visible to non-Zeta-using
users of inline completions.

Release Notes:

- Changed inline completions (Copilot, Supermaven, ...) to not show up
in empty buffers.

---------

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Bennet <bennet@zed.dev>

crates/editor/src/editor.rs

@@ -4511,7 +4511,8 @@ impl Editor {
         if !user_requested
             && (!self.enable_inline_completions
                 || !self.should_show_inline_completions(&buffer, cursor_buffer_position, cx)
-                || !self.is_focused(cx))
+                || !self.is_focused(cx)
+                || buffer.read(cx).is_empty())
         {
             self.discard_inline_completion(false, cx);
             return None;