add protection for when kernel_quantizer is None

[jmitrevs]

Description

The hls4ml software assumes that in QKeras, QDense, QConv, etc, will always have kernel_quantizer or similar defined. This fixes the case when they are not, as found in issue #990.

Type of change

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

Tests

Must first check that this doesn't break other tests. Whether we want to add a test for this particular situation I am not sure.

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.