unit test for source writer

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 42.77%. Comparing base (983c151) to head (2608dc5). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1749 +/- ## ============================================ + Coverage 42.15% 42.77% +0.61% - Complexity 3278 3684 +406 ============================================ Files 808 815 +7 Lines 47293 47678 +385 Branches 5053 5107 +54 ============================================ + Hits 19938 20394 +456 + Misses 25710 25627 -83 - Partials 1645 1657 +12 ``` | [Components](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=components&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [spanner-templates](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `63.06% <100.00%> (+1.59%)` | :arrow_up: | | [spanner-import-export](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `64.42% <ø> (-0.03%)` | :arrow_down: | | [spanner-live-forward-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `75.06% <ø> (+0.29%)` | :arrow_up: | | [spanner-live-reverse-replication](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `68.73% <100.00%> (+2.61%)` | :arrow_up: | | [spanner-bulk-migration](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/components?src=pr&el=component&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | `83.65% <ø> (+1.21%)` | :arrow_up: | | [Files](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform) | Coverage Δ | | |---|---|---| | [...leport/v2/templates/transforms/SourceWriterFn.java](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749?src=pr&el=tree&filepath=v2%2Fspanner-to-sourcedb%2Fsrc%2Fmain%2Fjava%2Fcom%2Fgoogle%2Fcloud%2Fteleport%2Fv2%2Ftemplates%2Ftransforms%2FSourceWriterFn.java&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform#diff-djIvc3Bhbm5lci10by1zb3VyY2VkYi9zcmMvbWFpbi9qYXZhL2NvbS9nb29nbGUvY2xvdWQvdGVsZXBvcnQvdjIvdGVtcGxhdGVzL3RyYW5zZm9ybXMvU291cmNlV3JpdGVyRm4uamF2YQ==) | `32.14% <100.00%> (+32.14%)` | :arrow_up: | ... and [24 files with indirect coverage changes](https://app.codecov.io/gh/GoogleCloudPlatform/DataflowTemplates/pull/1749/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GoogleCloudPlatform)