skip_missing is not working for fields of type String

[malexer]

It seems that skip_missing option is working only if field will have None value.
In case if input dict will not have some key which is declared as String type we will have and empty string in result.

Sample code:

class UserSchema(Schema):
    first = String()
    last = String()

    class Meta:
        skip_missing = True


test_data = dict(
    first='Name',
)

sch = UserSchema()
print sch.dump(test_data)

[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?