Avoid NPE as the Maven session can be null

[cdsap]

If MavenSession is null, calling Utils.projectProperty currently throws a NullPointerException:

java.lang.NullPointerException: Cannot invoke "org.apache.maven.execution.MavenSession.getUserProperties()" because "mavenSession" is null
    at com.gradle.Utils.projectProperty (Utils.java:33)
    at com.gradle.CustomBuildScanEnhancements.addCustomValueWhenProjectPropertyResolvesToTrue (CustomBuildScanEnhancements.java:476)
    at com.gradle.CustomBuildScanEnhancements.captureSkipTestsFlags (CustomBuildScanEnhancements.java:470)

This PR adds a simple null check to Utils.projectProperty. When the MavenSession argument is null, the method now returns Optional.empty() instead of throwing.

[cdsap]

I initially tried to provide parameterized tests to cover the different use cases, but I reverted that approach because the public MavenSession constructors are deprecated. Do you have an alternative?

[guylabs]

You might be able to add a test here

https://github.com/gradle/dv/blob/4ac9f78635f899e390fea59e9162bd520ec795b2/common-custom-user-data-maven-extension-test-func/src/test/groovy/com/gradle/maven/custom/CommonCustomUserDataMavenExtensionFuncTest.groovy#L39

And use this setup (which will run the build with a null Maven session) and see that it does not fail and publishes a Build Scan.

[guylabs]

You might be able to add a test here

https://github.com/gradle/dv/blob/4ac9f78635f899e390fea59e9162bd520ec795b2/common-custom-user-data-maven-extension-test-func/src/test/groovy/com/gradle/maven/custom/CommonCustomUserDataMavenExtensionFuncTest.groovy#L39

And use this setup (which will run the build with a null Maven session) and see that it does not fail and publishes a Build Scan.