Statistics tab update for large sequences

The behaviour of the groups on Statistics tab is different:

1. Open quite large sequence (e.g. chr1)
2. Open Statistics tab
   Current state: 'Common Statistics' group is opened and we can see the labels with default zero values, after the calculation is done the numbers are updated. But the 'Characters Occurrence' is empty on opening and the labels are visible only after the calculation is completed.
   The same happens when you expand 'Dinucleotides' group.

Make the behaviour consistent and change the default values to 'N/A'.