Fix BuildData cleanup script in documentation (jenkinsci#3895)

* Get rid of null keys

* Fix Groovy method override

Author: balakine

README.adoc

@@ -1633,6 +1633,7 @@ This script removes the static list of BuildsByBranch that is stored for each bu
 ----
 import hudson.matrix.*
 import hudson.model.*
+import static hudson.Util.fixNull
 
 hudsonInstance = hudson.model.Hudson.instance
 jobNames = hudsonInstance.getJobNames()
@@ -1660,7 +1661,7 @@ for (job in allItems) {
         hudson.plugins.git.Revision r = action.getLastBuiltRevision();
         if (r != null) {
           for (branch in r.getBranches()) {
-            action.buildsByBranchName.put(branch.getName(), action.lastBuild)
+            action.buildsByBranchName.put(fixNull((String) branch.getName()), action.lastBuild)
           }
         }
         build.actions.remove(action)
@@ -1679,7 +1680,7 @@ for (job in allItems) {
             hudson.plugins.git.Revision r = action.getLastBuiltRevision();
             if (r != null) {
               for (branch in r.getBranches()) {
-                action.buildsByBranchName.put(branch.getName(), action.lastBuild)
+                action.buildsByBranchName.put(fixNull((String) branch.getName()), action.lastBuild)
               }
             }
             run.actions.remove(action)