AVRO-2881: Fix Protobuf double default value (#927)

Default value of double should be 0.0 not 0
diff --git a/lang/java/protobuf/src/main/java/org/apache/avro/protobuf/ProtobufData.java b/lang/java/protobuf/src/main/java/org/apache/avro/protobuf/ProtobufData.java
index 38f2d04..ca4d78b 100644
--- a/lang/java/protobuf/src/main/java/org/apache/avro/protobuf/ProtobufData.java
+++ b/lang/java/protobuf/src/main/java/org/apache/avro/protobuf/ProtobufData.java
@@ -352,7 +352,9 @@
     case BOOL:
       return NODES.booleanNode(false);
     case FLOAT:
+      return NODES.numberNode(0.0F);
     case DOUBLE:
+      return NODES.numberNode(0.0D);
     case INT32:
     case UINT32:
     case SINT32: