Control use of `months` constants (for `JANUARY = 0` problem)

[MizukiTemma]

Motivation

We have already experienced two bugs which were related to months.JANUARY = 0 (#2598 and #3310). We should investigate whether there are other potential bugs due to this cause.

Proposed Solution

• Check for use of months constants in the repository, expecially in context of regions and MT budget
• If any bug prone points are found , fix them
• Add tests to code lines with months constants (if applicable), even if they are currently working fine.

Alternatives

User Story

Additional Context

Design Requirements

Related Issues

[JoeyStk]

Proposed solution: Identify all the if-statements not explicitly checking against null. Repair them. Then come up with a long term solution so this cannot happen again