fix: 'drop (stream|table) if exists' fails if source does not exist

[spena]

Description

What behavior do you want to change, why, how does your patch achieve the changes?
Fixes #4473

DROP STREAM <stream> should fail if does not exist.
DROP STREAM IF EXISTS <stream> should succeed if does not exist.

Testing done

Describe the testing strategy. Unit and integration tests are expected for any behavior changes.
Updated unit tests.
Verified manually

$ curl -X POST http://localhost:8088/ksql \
>      -H "Content-Type: application/vnd.ksql.v1+json; charset=utf-8" \
>      -d @<(cat <<EOF
> {
>   "ksql": "drop stream if exists test;",
>   "streamsProperties": {}
> }
> EOF
> )
[{"@type":"currentStatus","statementText":"drop stream if exists test;","commandId":"stream/TEST/drop","commandStatus":{"status":"SUCCESS","message":"Source `TEST` does not exist."},"commandSequenceNumber":3,"warnings":[]}]

$ curl -X POST http://localhost:8088/ksql \
>      -H "Content-Type: application/vnd.ksql.v1+json; charset=utf-8" \
>      -d @<(cat <<EOF
> {
>   "ksql": "drop stream test;",
>   "streamsProperties": {}
> }
> EOF
> )
{"@type":"statement_error","error_code":40001,"message":"Source TEST does not exist.","stackTrace":[],"statementText":"drop stream test;","entities":[]}

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[big-andy-coates]

Thanks @spena

Is it possible to add a QTT test covering both the positive and negative test cases? Not sure if QTT will support them... I think you should be able to do the negative test case at least.