perf(environments): skip stdin staging for empty payloads

execute() can pass stdin_data="" (e.g. write_file of empty content). The
`is not None` guard then paid an upload (plus chmod on Daytona) and a
longer shell command just to feed zero bytes. Base heredoc mode skipped
empty stdin, and neither SDK exec attaches a stdin, so treating "" as no
stdin gives exactly the base command (checked: identical argv, no upload).

tools/environments/daytona.py

@@ -162,7 +162,7 @@ class DaytonaEnvironment(BaseEnvironment):
 
         def exec_fn() -> tuple[str, int]:
             command = cmd_string
-            if stdin_data is not None:
+            if stdin_data:  # empty stdin == no stdin, as on base (heredoc skipped it)
                 with lock:
                     if state["cancelled"]:
                         return ("", 130)

tools/environments/vercel_sandbox.py

@@ -365,7 +365,7 @@ class VercelSandboxEnvironment(BaseEnvironment):
 
         def exec_fn() -> tuple[str, int]:
             command = cmd_string
-            if stdin_data is not None:
+            if stdin_data:  # empty stdin == no stdin, as on base (heredoc skipped it)
                 with lock:
                     if state["cancelled"]:
                         return ("", 130)