made ImageLoaderTask.TryLoadFromMemoryCacheAsync null safe

[thisisthekap]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Fixes #1523

:arrow_heading_down: What is the current behavior?

NullReferenceException thrown, when ImageLoaderTask.MemoryCache has entries with null values.

:new: What is the new behavior (if this is a feature change)?

No NRE anymore, just reporting it as cache miss.

:boom: Does this PR introduce a breaking change?

No.

:bug: Recommendations for testing

None, as #1523 was reported by our production error reporting, and unfortunately no reproduction steps are available.

:memo: Links to relevant issues/docs

None

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines
• [ ] Relevant documentation was updated
• [x] Rebased onto current develop

[thisisthekap]

@daniel-luberda The failing AppVeyor build seems to be unrelated to my change.

[thisisthekap]

@daniel-luberda Is my change breaking the CI?

[thisisthekap]

@daniel-luberda @molinch The build seems to be broken since https://ci.appveyor.com/project/daniel-luberda/ffimageloading/builds/35814199

[thisisthekap]

@daniel-luberda @molinch Can you provide any schedule when this PR is going to be processed?

[artemtishchenkobinwell]

Same problem

[thisisthekap]

@artemtishchenkobinwell I provided a fix in this PR, but unfortunately, I do not know if or when this PR will get reviewed.

[SelyutinSergey]

Same problem

[maukur]

Same problem

[thisisthekap]

@artemtishchenkobinwell @SelyutinSergey @maukur If you feel fearless, you might build your custom nuget with my fix included. Of course without any warranty.

[thisisthekap]

@daniel-luberda @molinch Is this PR going to get reviewed, or should it be closed?

[thisisthekap]

I'll keep that PR open for fellow developers who are stumbling upon the related issue. As this repository seems to be abandoned, there is no further action taken from my side.