Fixes an issue where ScheduleTypeLimits would not serialize to IDF strings when attributes are None

[samuelduchesne]

Before this fix, the following would error out on the __repr__ method of EpBunch objects because UpperLimit is None.

from archetypal.schedule import Schedule, ScheduleTypeLimits
from archetypal import IDF

idf = IDF()
# Create the ScheduleTypeLimits object
typelimits = ScheduleTypeLimits(Name="Test", LowerLimit=0, UpperLimit=None)
typelimits.to_epbunch(idf)

[samuelduchesne]

superseded by #498