fix: skip empty input queries to prevent sending blank messages to agent

Added validation to ignore when user presses Enter on an empty line,
preventing blank queries from being sent to the agent. This improves
UX by matching expected behavior where empty input is simply ignored
and the prompt is shown again.

The check validates input after command routing but before processing,
catching empty queries from single-line input, templates, and as a
defensive check for multiline input (which already has its own check).

Fixes: Users pressing Enter on empty line no longer sends to agent

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jwesleye

src/basic_agent_chat_loop/chat_loop.py

@@ -1155,6 +1155,10 @@ async def _async_run(self):
                         CommandType.TEMPLATE,
                         CommandType.MULTILINE,
                     ):
+                        # Skip empty queries
+                        if not user_input.strip():
+                            continue
+
                         # Process query through agent
                         logger.info(f"Processing query: {user_input[:100]}...")