MDEV-15420 Wrong result for CAST from TIME or DATETIME with zero integer part and non-zero microseconds to DECIMAL(X,Y)

The loop in ull2dec() does not iterate if "from" is zero,
so to->intg got erroneously set to 0 instead of 1.
Because if this, my_decimal2seconds() wrote the fractional
part into a wrong buf[x].

Catching the special case with zero "from" and properly initialize "to"
using decimal_make_zero().