Added a null check since the current version fails on Java 1.7 which does not allow null additions on a TreeSet.

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1493004 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/dependencymanager/core/src/main/java/org/apache/felix/dm/impl/index/multiproperty/Property.java b/dependencymanager/core/src/main/java/org/apache/felix/dm/impl/index/multiproperty/Property.java
index e4b466d..4e7c683 100644
--- a/dependencymanager/core/src/main/java/org/apache/felix/dm/impl/index/multiproperty/Property.java
+++ b/dependencymanager/core/src/main/java/org/apache/felix/dm/impl/index/multiproperty/Property.java
@@ -59,9 +59,12 @@
 			m_valid = false;
 		}
 		if (this.m_value == null) {
+			// value has not bee set yet
 			this.m_value = value;
 		}
-		m_values.add(value);
+		if (value != null) {
+			m_values.add(value);
+		}
 	}
 	
 	public boolean isNegate() {