Increase coverage for new helper function (#1141)

## Changes
<!-- Summary of your changes that are easy to understand. Add
screenshots when necessary -->
The original PR got merged but was low on coverage, so adding a quick
fix to increase coverage

### Linked issues
#1072 

### Functionality 

- [ ] added relevant user documentation
- [ ] added new CLI command
- [ ] modified existing command: `databricks labs ucx ...`
- [ ] added a new workflow
- [ ] modified existing workflow: `...`
- [ ] added a new table
- [ ] modified existing table: `...`

### Tests
<!-- How is this tested? Please see the checklist below and also
describe any other relevant tests -->

- [ ] manually tested
- [x] added unit tests
- [ ] added integration tests
- [ ] verified on staging environment (screenshot attached)

Author: pritishpai

tests/unit/test_install.py

@@ -1567,3 +1567,6 @@ def test_extract_major_minor_versions():
 
     assert extract_major_minor(version_string1) == "0.3"
     assert extract_major_minor(version_string2) == "0.17"
+
+    version_string3 = "should not match"
+    assert extract_major_minor(version_string3) is None