Fix Identify Prerelease step in the release worflow

[MatteoPologruto]

The condition to identify a prerelease was recently split to reduce the length of the line, causing the workflow to fail throwing this error:

/home/runner/work/_temp/1435c811-653a-480e-bcf6-c38ae8bd5d7e.sh: line 7: unexpected token `newline', conditional binary operator expected
Error: Process completed with exit code 2.

Adding some \ inside the condition should fix this issue.
This PR is also introducing an explicit comparison inside the test, which checks whether semver returns a string or not, to make the code more readable.

[per1234]

Thanks Matteo!