Fix division by zero error in rack space calculation. (functions: getRSUforRack, getRSUforRow)

[MarcHagen]

Previously, a division by zero occurred when the rack was empty and no rack space was available. This change adds a condition to return 0 when the total counter value is zero, preventing the error.

[infrastation]

Thank you for preparing this bug fix. What are the steps to reproduce the problem?

[MarcHagen]

1. Create a new row (any props is fine)
2. Create a new rack (any props is fine)
3. On the newly created rack go to Design
4. Unselect all the rows/columns in the Rack design
5. Hit the Set rack design button
6. Profit with 500 error on the rack page
7. Profit with 500 error on the view row page (will only happen if the one or all racks have no design, if there is a second rack that has selected design options it will show the page correctly.)

[infrastation]

Thank you, let me have a closer look.