Fix weight_exp is None for netx DelaySynapse

[Michaeljurado42]

Issue Number: #315

Objective of pull request:

Fix weight_exp being None for delay Synapses in netx converted models.

Pull request checklist

Your PR fulfills the following requirements:

• [x] Issue created that explains the change and why it's needed
• [x] Tests are part of the PR (for bug fixes / features)
• [x] Docs reviewed and added / updated if needed (for bug fixes / features)
• [x] PR conforms to Coding Conventions
• [x] PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• [x] Lint (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• [x] Build tests (pytest) passes locally

Pull request type

Please check your PR type:

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation changes
• [ ] Other (please describe):

What is the current behavior?

Weight_exp is always None (which is equivalent to it being zero)

What is the new behavior?

Weight exp is set to whatever optimize_weight_bits returns

Does this introduce a breaking change?

• [] Yes
• [x] No

Supplemental information

Very small fix that does not affect unit tests