fix(memory): ignore empty consolidation summaries

tests/agent/test_context_prompt_cache.py

@@ -96,6 +96,17 @@ def test_unprocessed_history_injected_into_system_prompt(tmp_path) -> None:
     assert re.search(r"\[\d{4}-\d{2}-\d{2} \d{2}:\d{2}\]", prompt)
 
 
+def test_nothing_history_entry_is_not_injected(tmp_path) -> None:
+    workspace = _make_workspace(tmp_path)
+    builder = ContextBuilder(workspace)
+    builder.memory.append_history("(nothing)")
+
+    prompt = builder.build_system_prompt()
+
+    assert "# Recent History" not in prompt
+    assert "(nothing)" not in prompt
+
+
 def test_recent_history_injection_is_session_scoped(tmp_path) -> None:
     workspace = _make_workspace(tmp_path)
     builder = ContextBuilder(workspace)