fix: ignore reward submissions with a duration of 0

[seanmcgary]

Found during the rewards v2.1 audit, this prevents a potential division by 0 case since non-zero durations are not enforced by the RewardsCoordinator.

Type of change

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

How Has This Been Tested?

Added additional EigenState model tests.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• 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 added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings