Improve nullability treatment for PageFlowUtil.urlProvider()

[labkey-jeckels]

Rationale

The vast majority of callers of PageFlowUtil.urlProvider() are in the module that supplies the provider, or in a module that has a hard dependency on the supplying module. We can eliminate thousands of warnings in the code about null return values by differentiating between nullable and non-nullable use cases.

Changes

• Split PageFlowUtil.urlProvider() (throws a clear exception instead of returning null) and PageFlowUtil.urlProviderOptional() (may return null)
• Update select callers to eliminate their own null special-casing
• Minor code cleanup

Tasks 📍

• Manual Testing
• Test Automation
• Verify Fix