Adding tests on the modified signal of the database when merging. This is to make sure that the modified signal is not emitted when nothing was performed during the merge operation.

Motivation and context

The previous implementation of the merge function was always emitting the modified signal.

Types of changes

• ✅ other (new tests)

Checklist:

• ✅ I have read the CONTRIBUTING document. [REQUIRED]
• ✅ My code follows the code style of this project. [REQUIRED]
• ✅ All new and existing tests passed. [REQUIRED]
• ✅ I have added tests to cover my changes.