support removing empty values in ListOf

[willkg]

ListOf takes a sub parser and an optional delimiter argument denoting what the delimiter for the items in the value should be.

That gives us things like this:

>>> from everett.manager import ListOf
>>> ListOf(str)("a, b,c")
['a', 'b', 'c']
>>> ListOf(str)("a, b,,,c")
['a', 'b', '', '', 'c']
>>> 

The problem is the empty strings which require the developer to add additional value validation/adjustments. Notably, if they don't want to support empty values (it's likely a typo), they have to do something like this:

>>> [item for item in ListOf(str)("a, b,,,c") if item]
['a', 'b', 'c']

I think we want to allow the developer to denote that empty strings are a configuration error.

[willkg]

I think we have a couple of options. We could have ListOf be in charge of whether an empty string is a valid item before sending it to the sub-parser:

ListOf(str, allow_empty=False)

Or we could have the sub-parser be in charge of whether the item is valid:

ListOf(nonempty(str))

I think I may do the latter. That would be usable in other places, too.

[willkg]

Scratch that. If we do the former, we can get a better error message because ListOf can say something like "yo, this configuration value 'a,,b' has empty strings in it" rather than "yo, this '' is an empty string".