fix: Validate when GSTIN field is empty

[Ninad1306]

Issue Reference: https://support.frappe.io/helpdesk/tickets/22783

_{Huly®: IC-2767}

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 60.05%. Comparing base (8b93f8a) to head (5c8d7e2). Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../gst_india/overrides/subcontracting_transaction.py	0.00%	2 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/resilient-tech/india-compliance/pull/2643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=resilient-tech) ```diff @@ Coverage Diff @@ ## develop #2643 +/- ## ======================================== Coverage 60.05% 60.05% ======================================== Files 120 120 Lines 11572 11572 ======================================== Hits 6949 6949 Misses 4623 4623 ``` | [Files with missing lines](https://app.codecov.io/gh/resilient-tech/india-compliance/pull/2643?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=resilient-tech) | Coverage Δ | | |---|---|---| | [.../gst\_india/overrides/subcontracting\_transaction.py](https://app.codecov.io/gh/resilient-tech/india-compliance/pull/2643?src=pr&el=tree&filepath=india_compliance%2Fgst_india%2Foverrides%2Fsubcontracting_transaction.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=resilient-tech#diff-aW5kaWFfY29tcGxpYW5jZS9nc3RfaW5kaWEvb3ZlcnJpZGVzL3N1YmNvbnRyYWN0aW5nX3RyYW5zYWN0aW9uLnB5) | `29.06% <0.00%> (ø)` | | [](https://app.codecov.io/gh/resilient-tech/india-compliance/pull/2643?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=resilient-tech)