Dynamically remove null value from swift dictionary using function

Rather than using a global function (or a method), why not making it a method of Dictionary, using an extension?

extension Dictionary {
    func nullKeyRemoval() -> Dictionary {
        var dict = self

        let keysToRemove = Array(dict.keys).filter { dict[$0] is NSNull }
        for key in keysToRemove {
            dict.removeValue(forKey: key)
        }

        return dict
    }
}

It works with any generic types (so not limited to String, AnyObject), and you can invoke it directly from the dictionary itself:

var dic : [String: AnyObject] = ["FirstName": "Anvar", "LastName": "Azizov", "Website": NSNull(),"About": NSNull()]
let dicWithoutNulls = dic.nullKeyRemoval()

Swift 5 adds compactMapValues(_:), which would let you do

let filteredDict = dict.compactMapValues { $0 is NSNull ? nil : $0 }

For Swift 3.0 / 3.1 this could be helpful. Also removes NSNull objects recursive:

extension Dictionary {
    func nullKeyRemoval() -> [AnyHashable: Any] {
        var dict: [AnyHashable: Any] = self

        let keysToRemove = dict.keys.filter { dict[$0] is NSNull }
        let keysToCheck = dict.keys.filter({ dict[$0] is Dictionary })
        for key in keysToRemove {
            dict.removeValue(forKey: key)
        }
        for key in keysToCheck {
            if let valueDict = dict[key] as? [AnyHashable: Any] {
                dict.updateValue(valueDict.nullKeyRemoval(), forKey: key)
            }
        }
        return dict
    }
}