Fix NPE in UpgradeDependencyVersion ext property upgrade when version is unresolvable (#7299)

DependencyVersionSelector.select() can return null when no matching version
is found. The downgrade prevention code added in #7290 passes this null into
versionComparator.upgrade() via singletonList(selectedVersion), which then
calls RELEASE_PATTERN.matcher(null) and throws NullPointerException.

Add a null guard consistent with the pattern used elsewhere in the same file
(e.g., line 480).

Fixes downstream failure in moderneinc/rewrite-spring.

Author: Jenson3210

rewrite-gradle/src/main/java/org/openrewrite/gradle/UpgradeDependencyVersion.java

@@ -583,6 +583,9 @@ public J postVisit(J tree, ExecutionContext ctx) {
                                         }
                                         selectedVersion = dependencyVersionSelector.select(ga, "classpath", newVersion, versionPattern, execCtx);
                                     }
+                                    if (selectedVersion == null) {
+                                        return prop.getTree();
+                                    }
                                     VersionComparator versionComparator = getVersionComparator();
                                     if (versionComparator != null) {
                                         Optional<String> finalVersion = versionComparator.upgrade(prop.getValue(), singletonList(selectedVersion));