Previously, `Num.fromString` accepted strings like `"1.2prefix"` and returned the
leading numeric portion (`1.2`). This change makes parsing stricter by requiring
that the entire string be consumed after `strtod`, skipping only trailing
whitespace. If any non-whitespace characters remain after the number, the
function now returns `null` instead of a partial result.
The fix also adds an explicit empty-string check and updates the test suite to
reflect the new behavior, replacing the old `"1.2prefix"` success test with a
`"1.2suffix"` null-return test.
Bob Nystrom
3aa82e7960