web_server.py: return empty content when file doesn't exist (#5980)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/dashboard/web_server.py

@@ -792,13 +792,22 @@ class EditRequestHandler(BaseHandler):
         """Get the content of a file."""
         loop = asyncio.get_running_loop()
         filename = settings.rel_path(configuration)
-        content = await loop.run_in_executor(None, self._read_file, filename)
+        content = await loop.run_in_executor(
+            None, self._read_file, filename, configuration
+        )
+        if content is not None:
             self.write(content)
 
-    def _read_file(self, filename: str) -> bytes:
+    def _read_file(self, filename: str, configuration: str) -> bytes | None:
         """Read a file and return the content as bytes."""
+        try:
             with open(file=filename, encoding="utf-8") as f:
                 return f.read()
+        except FileNotFoundError:
+            if configuration in const.SECRETS_FILES:
+                return ""
+            self.set_status(404)
+            return None
 
     def _write_file(self, filename: str, content: bytes) -> None:
         """Write a file with the given content."""