Added accommodations for empty dataframes in diffusive

[JurgenZach-NOAA]

Dataframes added/modified for diffusive & coastal routing threw error in some corner cases

Additions

• accommodation for empty self._network._nexus_latlon in troute_model
• accommodation for empty waterbodies df in model_DAforcing
• accommodation for empty nudge df in model_DAforcing

Removals

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Changes

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Testing

1.

Screenshots

Notes

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Todos

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Checklist

• [ ] PR has an informative and human-readable title
• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows project standards (link if applicable)
• [ ] Passes all existing automated tests
• [ ] Any change in functionality is tested
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged / future todos are captured in comments
• [ ] Visually tested in supported browsers and devices (see checklist below :point_down:)
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [ ] Reviewers requested with the Reviewers tool :arrow_right:

Testing checklist

Target Environment support

• [ ] Windows
• [ ] Linux
• [ ] Browser

Accessibility

• [ ] Keyboard friendly
• [ ] Screen reader friendly

Other

• [ ] Is useable without CSS
• [ ] Is useable without JS
• [ ] Flexible from small to large screens
• [ ] No linting errors or warnings
• [ ] JavaScript tests are passing