Don't crash if null value passed in parameters map

If a null String value is passed into the map, Flurry will crash. Replace null values with an empty string, and replaces keys too just in case. (The irony of causing a crash because of passing something to Flurry to report on it should be avoided!)
Merge pull request #91 from mikemee/patch-1

Don't crash if null value passed in parameters map.