chore(iast): django Invalid or empty source_value [backport 2.13]

[avara1986]

Backport 26d9c60 from #10816 to 2.13.

Fix telemetry log error: "ValueError: [IAST] Invalid or empty source_value."

No release note needed, as this error occurs after this PR #10740 that has not been released yet

Task APPSEC-55017

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

.gitlab/tests/appsec.yml                                                @DataDog/asm-python
ddtrace/appsec/_handlers.py                                             @DataDog/asm-python
ddtrace/appsec/_iast/_metrics.py                                        @DataDog/asm-python
ddtrace/appsec/_iast/_taint_tracking/__init__.py                        @DataDog/asm-python
tests/appsec/iast/conftest.py                                           @DataDog/asm-python
tests/appsec/iast/test_processor.py                                     @DataDog/asm-python
tests/appsec/iast/test_telemetry.py                                     @DataDog/asm-python
tests/contrib/django/django_app/appsec_urls.py                          @DataDog/asm-python
tests/contrib/django/test_django_appsec_iast.py                         @DataDog/asm-python
tests/contrib/fastapi/test_fastapi_appsec_iast.py                       @DataDog/asm-python

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: backport-10816-to-2.13
Commit report: 6a552ea
Test service: dd-trace-py

✅ 0 Failed, 2199 Passed, 212 Skipped, 1m 28s Total duration (1m 50.61s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-09-27 08:42:59

Comparing candidate commit 6a552ea in PR branch backport-10816-to-2.13 with baseline commit 6c3d192 in branch 2.13.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 353 metrics, 47 unstable metrics.