fix(ios/core): filter out nil values from serialize method

[mukaschultze]

Firebase analytics' logEvent method fails on iOS when there's an array with null/undefined elements and when objects have null/undefined prop values.

With this PR the null elements are filtered out of arrays and dictionaries.

This PR also fixes a slight oversight with data.map(serialize), since this syntax causes the second argument to be the element index instead of wrapPrimitives, causing the first element not to be wrapped and all the others to be.

firebase().analytics().logEvent('eventname', [ null, undefined ]);
// Error: *** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0] Error: *** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[1]

firebase().analytics().logEvent('eventname', { someProp: null, otherProp: undefined });
// Error: *** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0] Error: *** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[0]

firebase-database might have the same issue since it declares its own version of the serialize method, but I haven't tested it.