[SEDONA-137] Fix ST_Buffer for Flink to work

[sekikn]

Did you read the Contributor Guide?

• Yes, I have read Contributor Rules and Contributor Development Guide

Is this PR related to a JIRA ticket?

• Yes, the URL of the assoicated JIRA ticket is https://issues.apache.org/jira/browse/SEDONA-137. The PR name follows the format [SEDONA-XXX] my subject.

What changes were proposed in this PR?

Currently, the ST_Buffer function for Flink is broken. This PR fixes it.

How was this patch tested?

Ran mvn test -pl flink locally and ensured all tests passed including the new one.

Did this PR include necessary documentation updates?

• No, this PR does not affect any public API so no need to change the docs.

[jiayuasu]

To be merged after this PR: https://github.com/apache/incubator-sedona/pull/647