Fix crash for method cacheKeyForURL: when url is nil

[NSKevin]

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This merge request fixes / reffers to the following issues: ...

Pull Request Description

In method cacheKeyForURL, when the url was nil, the app crashed. So I added a safe check on it.

[bpoplauschi]

@NSKevin thanks for this. I adapted a bit your fix. Since we have places where we use that key as a dictionary key, I preferred to return an empty string rather than nil when the url was nil. 1bf62d4