Drop default for @DefaultValue value.

Author: greenrobot-team

objectbox-java-api/src/main/java/io/objectbox/annotation/DefaultValue.java

@@ -14,5 +14,5 @@
 @Retention(RetentionPolicy.CLASS)
 @Target({ElementType.FIELD})
 public @interface DefaultValue {
-    String value() default "";
+    String value();
 }