fix: edge case construcing from string '0'

[natenichols]

High Level Overview of Change

Handles the edge case where UInt64 is constructed from string '0'.

Context of Change

When constructing UInt64 with.from(), this adds an edge case when the parameter is of type string where UInt64.from('0') returnsUInt64.from(0).

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [x] Tests (You added tests for code that already exists, or your new feature included in this PR)
• [ ] Documentation Updates
• [ ] Release

Test Plan

Added test coverage for this case.

[intelliot]

Looks good... could we also add a test case that uses some realistic-ish data from rippled?

Since this is covering a case that only occurs as of rippled v1.7.0+