Bug: Error when ksampler receives no information (missing validation for null t in timestep_embedding)

## Bug Description
An error appears when `ksampler` does not receive information. Specifically, in `timestep_embedding` (comfy/ldm/flux/layers.py), there is no validation for `t` being null or None before multiplying with `time_factor`. This can result in an exception when `t` is not provided.

## Expected Behavior
`timestep_embedding` should validate that `t` is not null or None before proceeding with calculations (e.g., multiplication with `time_factor`).

## Steps to Reproduce
- Trigger `ksampler` with no information or missing arguments.
- Observe the error message generated due to `t` being null or None.

## Suggested Fix
Add input validation in `timestep_embedding` to ensure `t` is provided and is a valid tensor before performing calculations.

## Context
File: comfy/ldm/flux/layers.py
Commit: fe01885acf892de636b1b2743903812099bd42e3
Related function: `timestep_embedding`