Data producer paragraph_behavior is broken with NULL default value

[mrmishmash]

The paragraph behavior data producer as it is is broken. The default value can never be set to NULL, because if it is, the whole producer will fail and resolve to NULL, even when the behavior plugin is enabled and there is a value for the given key. Using NULL as the default value when there is no value is standard practice and as it is this cannot be done.

Throwing an exception when the plugin isn't enabled is also not ideal in my opinion. Let's say one wants to implement a behavior and fetch it on all paragraphs, even the ones that do not have it enabled, which is practical in order not to have to call it just on paragraph resolver that have it enabled, one can just enable it on a paragraph and it works if it's used in a i.e. base paragraph resolver.

Currently this isn't possible and would throw, to mitigate this, one can extend the producer and wrap the resolve() method in a try/catch block, but since paragraphs that do not have the behavior enabled will always throw, this leads to a significant performance hit because of the exception handling.

I suggest a parameter that allows the caller to determine if the producer should throw in such cases or not, with it being set to TRUE for backwards compatibility.

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