Don't allow empty/hex strings to be validated as numeric strings

[iamacook]

Summary

Our NumericStringSchema validates empty strings (0) and hexadecimal strings, but it shouldn't. This prevents empty/hex strings from passing validation.

Note: it also corrects mock data that was assumed hex, when it should be numeric.

Changes

• Prevent empty/hex strings from passing
• Fix mock data accordingly
• Add relevant test coverage