skip_missing is not working for fields of type String about marshmallow

[sloria]

Only key:value pairs for which value=None are skipped. This is intentional. marshmallow does not make assumptions about whether a falsy value--such as an empty string--is a "null" value or if it is a valid, user-provided value. We would not, for example, want to skip 0 for a Number field.

For now, you will have to explicitly set a String field's default to None for it to be skipped.

A more long-term solution would be to allow Field classes to define a set of null values that are skippable:

class String(Field):
    # values that are skipped when skip_missing=True
    NULL_VALUES = set([None, ''])

class List(Field):
    # can't use a set, because lists are unhashable
    NULL_VALUES = (None, [], tuple())

Thoughts?