Fix: Crash when getting statistical data without permissions

[GGGava]

Description

Lib currently crashes when trying to get statistical data without user permission. This happens because startDate and endDate fields on the query's response are null in this case, and the code crashes when trying to add them to an NSDictionary. Fixed by returning an error to js.

Fixes #97

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have checked my code and corrected any misspellings