Add second check for position values (iOS)

[JamesCrowMedia]

This PR resolves #295

What was causing the issue?

In www/plugins.FileOpener2.js:

When calling showOpenWithDialog without an optional callbackContext object, an empty object is set by default in the JS bridge:

line 37		callbackContext = callbackContext || {};

When the exec function is called, the missing position data is then referenced in the arguments array, resulting in an undefined value:

line 38		[fileName, contentType, false, callbackContext.position]

In src/ios/FileOpener2.m:

The undefined data is interpreted as a NSNull object and still placed in the array.

On line 86, there is a check for array length, but not a check to see if the object is valid. This means positionValues on line 87, which should be a 2 item array, is a NSNull object instead. When the expected array items are accessed on line 88, this causes a NSInvalidArgumentException.

line 86		if ([command.arguments count] >= 4) {
line 87			NSArray *positionValues = [command.arguments objectAtIndex:3];
line 88			rect = CGRectMake(0, 0, [[positionValues objectAtIndex:0] floatValue], [[positionValues objectAtIndex:1] floatValue]);

How does this PR correct this issue?

This PR adds a second check to the if statement on line 86 to make sure the item at index 3 in not NSNull.

&& ![[command.arguments objectAtIndex:3] isEqual: [NSNull null]]

[pwlin]

@JamesCrowMedia Thanks!