Interest: Display months without bookings and fix hardcoded opening balance interest

[nihas101]

This MR changes the behavior for the interest related displays as follows

• Months without bookings were previously hidden. They are now displayed
• The interest for the opening balance was previously hard-coded to 300, this caused incorrect calculations for the interest when the opening balance contributed more or less than exactly 300 to the sum of interest
• 0 Values are hidden in the interest and transaction rows, so those columns stay empty