Fixes 500 when depreciation is active but no purchase date

[snipe]

If a user had a license with a depreciation but no purchase date, the system would 500.

This adds validation to make sure that doesn't happen, and also more gracefully fails.

Oh, and tests.

[what-the-diff]

PR Summary

• Improved safety checks in depreciation calculations
  We've made an improvement in our depreciation computation where the system checks if a depreciation date is available before performing the calculation. Previously, the system could attempt a calculation without a date, leading to potential errors.

• Enforced purchase date requirement with depreciation
  The system has been updated to require 'purchase date' when a depreciation ID is provided. This ensures that all necessary data is available for accurate calculations.

• Expanded license management tests
  Two new test files have been created:

  1. A 'CreateLicenseTest' file that includes tests to verify that proper permissions are needed to view a license, and that the system correctly handles situations when a license is created without a purchase date.
  2. A 'LicenseViewTest' file that contains tests looking at license viewing permissions, how the system handles licenses without a purchase date, and ensuring that the depreciation calculations for a license with a purchase date are accurate.

These tests further strengthen the robustness of our software by proactively checking for potential issues.