add check for no inputs in insert_node

[jmitrevs]

Description

graph.insert_node() assumes all the nodes have an input, but constant nodes do not. This adds a check to not fail if there is a node with no input in the graph.

Type of change

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

Tests

None at the moment, though we have a script that caused this to fail before. We should discuss whether we should add it.

Checklist

• I have read the guidelines for contributing.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have installed and run pre-commit on the files I edited or added.
• I have added tests that prove my fix is effective or that my feature works.