fix(form): do not prevent submit when action attribute equals to an empty string

[agentcooper]

Closes #3370

[petebacondarwin]

@agentcooper - Thanks for the PR!
The content of the commit LGTM.
Can you ensure you have signed the CLA.
Also, the commit message subject is too long. Can you fix it up?

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Thanks again for your help!

[agentcooper]

Real name for the CLA: Artem Tyurin

[Narretz]

I'd like to bump this for one of the next releases, but we should add a test for new case in formSpec.js

[caitp]

I guess it looks okay, it's a bit of a breaking change but I don't think it's likely to hurt anyone -- personally I would just say if (!isString(attr.action)) though

[IgorMinar]

this is not right. the point of the test is to simulate conditions when the form is being submitted. which means that we need the action attribute set to something

[IgorMinar]

I fixed this PR and merged it in. thanks!