Fix build randomization to account for null strings (elastic#101019)

Since adding qualifier as a nullable string, the build utils which
mutates Build objects for tests may experience an NPE. This commit
handles null for string values.

closes elastic#100994

test/framework/src/main/java/org/elasticsearch/test/BuildUtils.java

@@ -100,6 +100,7 @@ public static Build mutateBuild(Build existing) {
     }
 
     private static String randomStringExcept(final String s) {
-        return randomAlphaOfLength(13 - s.length());
+        int len = s == null ? 0 : s.length();
+        return randomAlphaOfLength(13 - len);
     }
 }