fix: handle empty BWC versions stream in AbstractYamlRestCompatTestPlugin

When building an elasticsearch docker image without the full BWC version
checkout (e.g. the upgrade-elasticsearch workflow), getBwcVersions().getUnreleased()
may return an empty list for the previous major version (getMajor() == currentMajor-1).

The call to .min().get() on an empty stream throws NoSuchElementException 'No value present'.
Fix by using .orElse(null) and returning early when no compatible BWC version is found.

Author: CVE Bot

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/test/rest/compat/compat/AbstractYamlRestCompatTestPlugin.java

@@ -117,7 +117,12 @@ public void apply(Project project) {
             .stream()
             .filter(v -> v.getMajor() == currentMajor - 1)
             .min(Comparator.reverseOrder())
-            .get();
+            .orElse(null);
+        // If no unreleased previous-major versions exist, REST compat testing is not applicable
+        // (e.g. building a standalone docker image export without BWC version checkout)
+        if (lastMinor == null) {
+            return;
+        }
         String lastMinorProjectPath = buildParams.getBwcVersions().unreleasedInfo(lastMinor).gradleProjectPath();
 
         // copy compatible rest specs