Fix complex_graph_rewrite crashing on a None-valued placeholder

[jloftin-nv]

Description

Some models obtained from HF (i.e. Qwen-Image) have None placeholders, this fixes a crash for that

Fixes # (issue)

Type of change

Please delete options that are not relevant and/or add your own.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ X] My code follows the style guidelines of this project (You can use the linters)
• [ X] I have performed a self-review of my own code
• [X ] I have commented my code, particularly in hard-to-understand areas and hacks
• [X ] I have made corresponding changes to the documentation
• [X ] I have added tests to verify my fix or my feature
• [ X] New and existing unit tests pass locally with my changes
• [ X] I have added the relevant labels to my PR in so that relevant reviewers are notified