fix setting null value of property in xml throws NullPointerException (#2646)

* fix setting value of property in xml is null throws NullPointerException

* fix setting null value of property in xml throws NullPointerException

apollo-client/src/main/java/com/ctrip/framework/apollo/spring/property/PlaceholderHelper.java

@@ -4,6 +4,7 @@ import com.google.common.base.Strings;
 import com.google.common.collect.Sets;
 import java.util.Set;
 import java.util.Stack;
+import java.util.Objects;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.config.BeanExpressionContext;
@@ -65,7 +66,7 @@ public class PlaceholderHelper {
   public Set<String> extractPlaceholderKeys(String propertyString) {
     Set<String> placeholderKeys = Sets.newHashSet();
 
-    if (!isNormalizedPlaceholder(propertyString) && !isExpressionWithPlaceholder(propertyString)) {
+    if (Objects.isNull(propertyString) || (!isNormalizedPlaceholder(propertyString) && !isExpressionWithPlaceholder(propertyString))) {
       return placeholderKeys;
     }