[Releng] Try to get milestone updates to compose properly.

Tolerate missing drop properties.
diff --git a/releng/org.eclipse.oomph.releng/src/RepositoryComposer.java b/releng/org.eclipse.oomph.releng/src/RepositoryComposer.java
index 9a1123f..c46c7c1 100644
--- a/releng/org.eclipse.oomph.releng/src/RepositoryComposer.java
+++ b/releng/org.eclipse.oomph.releng/src/RepositoryComposer.java
@@ -141,7 +141,7 @@
     else
     {
       String folder = buildKey;
-      if (buildLabel.length() != 0)
+      if (buildLabel != null && buildLabel.length() != 0)
       {
         folder += "-" + buildLabel;
       }