Fix isEmpty() call

[ntkme]

This PR fixes the following error:

lib/src/value/number.ts:604:39 - error TS2774: This condition will always return true since this function is always defined. Did you mean to call it instead?

604       if (newNumerators.size === 1 && newDenominators.isEmpty) {
                                          ~~~~~~~~~~~~~~~~~~~~~~~


Found 1 error in lib/src/value/number.ts:604