fix: return error when the client transaction coordinator is nil to p…

[thomas-bousquet]

Motivation

When the Pulsar client is created, if EnableTransaction is true then the transaction coordinator client is created.

If an application tries to create a transaction with NewTransaction() when EnableTransaction is false, the application will panic; returning an error when the transaction coordinator is nil will prevent this.

Modifications

In NewTransaction(), a nil check is added . If the transaction coordination client is nil, the method now returns an error.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): no
• The public API: no
• The schema: no
• The default values of configurations: no
• The wire protocol: no

Documentation

• Does this pull request introduce a new feature? no

[crossoverJie]

Please add a unit test.

[thomas-bousquet]

Please add a unit test.

I added a test in the integration tests suite for transactions. I also exported the error, as it could be leveraged with errors.Is() by the applications creating the client.