fix(maven): Do not try to publish .module file when does not exist

[romtsn]

We have some repositories that do not use Gradle for publishing and therefore do not have the .module file, so we''re just checking for its presence before mentioning it to the ./mvnw gpg... command

[adinauer]

LGTM

[adinauer]

Thanks for fixing!
Here's the error log message that was in the log of the failing release job:

Error: ts/mvnw: [ERROR] Failed to execute goal org.apache.maven.plugins:maven-gpg-plugin:3.2.7:sign-and-deploy-file (default-cli) on project sentry-maven-plugin: Specified side artifact tmp/craft-5FNVwi/sentry-maven-plugin-0.1.0/sentry-maven-plugin-0.1.0.module does not exist -> [Help 1]

Here's a release that failed: getsentry/publish#4959