Allow diaper change without any contents

[myxor]

Fixes #426.

It looks like the model change is enough for this change.

[coveralls]

Coverage decreased (-0.03%) to 98.304% when pulling bf064a7 on myxor:426_diaper_without_contents into 0d89574 on babybuddy:master.

[cdubz]

This also affects thecard_diaperchange_types card but I'm not totally sure how best to address this change there. The idea of that card is to give a quick useful view of how often the child pee'd and pooped for the past few days (i.e., it's not necessarily about the changes themselves).

Just the total number of changes generally is not really as useful and if a user did not use wet or solid the card would just be empty. But maybe that's OK?

[myxor]

Maybe that's okay.
Another idea is that the card could contain a third bar called "empty" or "none" or something to represent the complete number of changes?

[cdubz]

Yeah but the "empty"/"none" group gets a little dicey because its also possible for a change to be wet and solid so you could have e.g. three diapers -- two wet and solid and one with none -- and the bar could end up look like 2 wet, 2 solid, 1 none. I guess that is fine though...

[myxor]

Would be fine for me.

[cdubz]

Merged! Thanks!