Fix NPE in UpgradeDependencyVersion ext property upgrade

[Jenson3210]

Summary

• Add null guard for selectedVersion in the ext property upgrade path of UpgradeDependencyVersion, fixing NPE when DependencyVersionSelector.select() returns null

Problem

• PR Prevent UpgradeDependencyVersion from downgrading ext property versions #7290 added downgrade prevention for ext property versions but passes selectedVersion (which is @Nullable) into versionComparator.upgrade() without a null check. When select() returns null (e.g., no matching version found), RELEASE_PATTERN.matcher(null) throws NullPointerException.

Stack trace from downstream CI:

java.lang.NullPointerException: Cannot invoke "java.lang.CharSequence.length()" because "this.text" is null
    at java.util.regex.Pattern.matcher(Pattern.java:1180)
    at org.openrewrite.semver.VersionComparator.checkVersion(VersionComparator.java:60)
    at org.openrewrite.semver.LatestRelease.isValid(LatestRelease.java:36)
    at org.openrewrite.semver.XRange.isValid(XRange.java:51)
    at org.openrewrite.semver.VersionComparator.upgrade(VersionComparator.java:46)
    at org.openrewrite.gradle.UpgradeDependencyVersion$UpdateGradle.lambda$postVisit$2(UpgradeDependencyVersion.java:588)

Fix

Add a null check for selectedVersion before the upgrade check, consistent with the same pattern used elsewhere in the file (e.g., line 480).

Test plan

• Existing UpgradeDependencyVersionTest.doesNotDowngradeExtPropertyVersion passes
• All UpgradeDependencyVersionTest tests pass
• All semver tests pass

[timtebeek]

Thanks for the quick fix!