Make it possible for users to disable .torchxconfig (meta-pytorch#1125)

Summary:

X-link: meta-pytorch#1125

So that the one below is valid and does not use any configs
```
TORCHXCONFIG="" torchx run
```

Reviewed By: kiukchung

Differential Revision: D83086473

Author: AbishekS

torchx/runner/config.py

@@ -494,6 +494,8 @@ def find_configs(dirs: Optional[Iterable[str]] = None) -> List[str]:
 
     config = os.getenv(ENV_TORCHXCONFIG)
     if config is not None:
+        if not config:
+            return []
         configfile = Path(config)
         if not configfile.is_file():
             raise FileNotFoundError(

torchx/runner/test/config_test.py

@@ -275,6 +275,12 @@ def test_get_configs(self) -> None:
             ),
         )
 
+    def test_no_config(self) -> None:
+        config_dir = self.tmpdir
+        with patch.dict(os.environ, {ENV_TORCHXCONFIG: str("")}):
+            configs = find_configs(dirs=[str(config_dir)])
+            self.assertEqual([], configs)
+
     def test_find_configs(self) -> None:
         config_dir = self.tmpdir
         cwd_dir = config_dir / "cwd"