Add missing nullable and nonnull attributes

[RomanPodymov]

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• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

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Pull Request Description

Hello.
Thank you for your library.
I found out that in some *.m-files nullable and nonnull attributes are missing. I know that this is not an issue because these attributes are using in the corresponding headers files. I just want to keep the same code style for all files.

[dreampiggy]

I know that this is not an issue because these attributes are using in the corresponding headers files

Yes. This is why we only mark all the public header's null ability, you can also find something in private header does not care about this.

Because, in Objective-C, the null ability is just a annotation without any compiler guide. So in the implementation files we does not care about this. It's only used for Swift Generated API and check.

Mark every implementation file will cause some dummy code change, I don't think it's a really important thing. But there are one thing we should add to our conttribution guideline.

• If one API is marked as **Public ** (must visible from outside and not in Private folder), the both interface and implementation should keep the nullability

This may be a better solution

[bpoplauschi]

@RomanPodymov Thanks a lot for showing interest and contributing to SDWebImage! I know it's been a while since you contributed, but we would like to invite you to become a maintainer – no pressure to accept! You can pitch in with what seems comfortable: comment on open issues/PRs, triage, improve documentation, write your own PRs. Let me know if you are interested.