Re-enabling empty_value python test

A recent change to removing obsolete values from time windows has resulted in the failure of python test test_empty_value.py and it was temporarily disabled in that change. See: WT-8270.

The intent of the test is to test that WiredTiger doesn't write out empty values with keys when writing them to disk. The change to the time window clearing code has result in WiredTiger writing out a validity window with the empty value and as such it cannot skip writing the empty value. We should fix the test so it writes out globally visible empty values.

Scope of work:

• Update the test to insert globally visible empty values.
• Confirm it passes.