use empty value if trunk.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@3737 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/scm/CVSSCM.java

@@ -728,8 +728,7 @@ public class CVSSCM extends SCM implements Serializable {
     public void buildEnvVars(AbstractBuild build, Map<String, String> env) {
         if(cvsRsh!=null)
             env.put("CVS_RSH",cvsRsh);
-        if(branch!=null)
-            env.put("CVS_BRANCH",branch);
+        env.put("CVS_BRANCH",Util.fixNull(branch));
         String cvspass = getDescriptor().getCvspassFile();
         if(cvspass.length()!=0)
             env.put("CVS_PASSFILE",cvspass);