Raise errors for validataclasses with missing type annotations

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Currently, the following code will raise an AttributeError because the class has no attribute __annotations__:

@validataclass
class ExampleClass:
    pass

You can replace the decorator with the usual @dataclass validator if you need an empty dataclass, but it would be more intuitive if the @validataclass decorator worked here too. So, simply check if __annotations__ exists and silently return if it doesn't.

... BUT there is a catch to this solution: It could hide potential errors and make debugging difficult.

@validataclass
class ExampleClass:
    some_field = IntegerValidator()

In this example, it's most likely an error that the field is missing the type annotation (some_field: int = IntegerValidator()). This would mean that the field doesn't even exist as a dataclass field at all. Currently, the AttributeError would be raised, so the error would be noticed.

So, better solution: Raise an error if __annotations__ is empty, but with a more userfriendly error message, and implement a parameter allow_empty that can be set to explicitly allow an empty dataclass (e.g. @validataclass(allow_empty=True).

Better solution: Detect fields with missing annotations directly, and raise errors about that. But only if the value of a field is of type Validator, Default or a tuple containing one of those (so that you can still define class variables without type annotations that are not supposed to be validated fields).

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Solved in #27.