Use valid schema related to null default based on specification in (#442)

Author: apupier

avro/src/test/java/com/fasterxml/jackson/dataformat/avro/schemaev/RecordEvolutionTest.java

@@ -17,7 +17,7 @@ public class RecordEvolutionTest extends AvroTestBase
         + " \"fields\": [\n"
         + "     {\n"
         + "        \"name\": \"name\",\n"
-        + "        \"type\": [\"string\", \"null\"],\n"
+        + "        \"type\": [\"null\", \"string\"],\n"
         + "        \"default\": null\n"
         + "    },\n"
         + "     {\n"
@@ -39,7 +39,7 @@ public class RecordEvolutionTest extends AvroTestBase
         + " \"fields\": [\n"
         + "     {\n"
         + "        \"name\": \"fullName\",\n"
-        + "        \"type\": [\"string\", \"null\"],\n"
+        + "        \"type\": [\"null\", \"string\"],\n"
         + "        \"default\": null,\n"
         + "        \"aliases\": [\"name\"]\n"
         + "    },\n"