Refactor QdrantObjectFactory to allow null metadata values

[WOONBE]

related issue : #3420
When retrieving documents from a Qdrant vector store, a NullPointerException was being thrown if the metadata payload for any document contained a field with a null value.

I fix the issue by replacing the stream-based Collectors.toMap() implementation with a more robust and explicit for-each loop.

I have confirmed the fix by performing tests in my local environment.

[ilayaperumalg]

@WOONBE Thanks for the PR! Could you add test(s) to validate the fix please?

[WOONBE]

@WOONBE Thanks for the PR! Could you add test(s) to validate the fix please?

@ilayaperumalg Regarding the unit test for this change, I couldn't find an existing test class for QdrantObjectFactory where this new test case would naturally fit.
Please let me know if I should create a new test file, QdrantObjectFactoryTests.java, or if there's a more suitable existing file you'd prefer I add it to. I'm happy to add the test case accordingly.

[ilayaperumalg]

@WOONBE Please feel free to add a new test class for the unit test. You could also add a test case in our ITs. Thanks again for looking into this!

[WOONBE]

@WOONBE Please feel free to add a new test class for the unit test. You could also add a test case in our ITs. Thanks again for looking into this!

@ilayaperumalg Thank you for the guidance!

I've pushed a new commit that adds a test class with a unit test to cover this fix.

Please let me know if there are any improvements or if the format needs any adjustments. I'd be happy to make further changes.

[WOONBE]

@ilayaperumalg My apologies, it seems I missed running the code formatter before submitting the initial PR.

I saw the "Formatting violations" error in the PR checks for the test file I added. I have now run mvn spring-javaformat:apply and pushed the updated code to resolve this.

Aside from this, please let me know if there's anything else I might have missed or any other feedback you have.
Thank you!

[ilayaperumalg]

Rebased and merged as 8b45f41. Also, cherry-picked into 1.0.x via 8053ae3

[ilayaperumalg]

@WOONBE Thank you once again for the PR and following it up with the test update.