fix: support empty string literals in textfx

- Fix tokenizer to emit empty string token when closing quote on empty buffer
- Add force parameter to flush_word() to handle empty quoted strings
- Add test case for echo "" and echo ''

tests/test_textfx_shell.py

@@ -205,3 +205,21 @@ async def test_while_body_can_use_if(runner: PipelineRunner):
     assert not isinstance(parsed, str)
     results = await runner.run_pipeline(parsed, None, TextHandlerEnvironment(False))
     assert results[0].code == 1
+
+
+@pytest.mark.asyncio
+async def test_echo_empty_string(runner: PipelineRunner):
+    """测试 echo 空字符串"""
+    # 双引号空字符串
+    parsed = runner.parse_pipeline('echo ""')
+    assert not isinstance(parsed, str)
+    results = await runner.run_pipeline(parsed, None, TextHandlerEnvironment(False))
+    assert results[0].code == 0
+    assert results[0].ostream == ''
+    
+    # 单引号空字符串
+    parsed2 = runner.parse_pipeline("echo ''")
+    assert not isinstance(parsed2, str)
+    results2 = await runner.run_pipeline(parsed2, None, TextHandlerEnvironment(False))
+    assert results2[0].code == 0
+    assert results2[0].ostream == ''