Objects.isNull is only available from jdk 1.8 but apollo-client is targeted to...

Objects.isNull is only available from jdk 1.8 but apollo-client is targeted to jdk 1.7+, so we need to use a compatible one (#2677)

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

@@ -66,7 +66,7 @@ public class PlaceholderHelper {
   public Set<String> extractPlaceholderKeys(String propertyString) {
     Set<String> placeholderKeys = Sets.newHashSet();
 
-    if (Objects.isNull(propertyString) || (!isNormalizedPlaceholder(propertyString) && !isExpressionWithPlaceholder(propertyString))) {
+    if (Strings.isNullOrEmpty(propertyString) || (!isNormalizedPlaceholder(propertyString) && !isExpressionWithPlaceholder(propertyString))) {
       return placeholderKeys;
     }