[Bug] validate_frame silently accepts empty payload (size == 0)

[harshithabair47-pixel]

Summary

validate_frame() in sigflow/validators/frame.py does not
reject frames with an empty payload (size == 0), allowing
invalid frames to pass validation silently.

Reproduction

1. Create a Frame with payload = b""
2. Call validate_frame(frame)
3. No ValidationError is raised

Expected Behavior

A frame with empty payload should raise ValidationError
as it carries no useful data and indicates a malformed frame.

Actual Behavior

frame.size returns 0 which passes all checks silently.

Environment

• Python version: 3.10+
• sigflow version: main
• Operating system: Windows

Additional Context

frame.size is a property returning len(self.payload).
The validator checks size > max_payload but never checks
size == 0, allowing empty frames into the pipeline.