Added tests for malformed mappings with no root object

This commit also makes the error message more consistent with
other exception messages in the DocumentMapperParser.

src/main/java/org/elasticsearch/index/mapper/DocumentMapperParser.java

@@ -241,14 +241,17 @@ public class DocumentMapperParser extends AbstractIndexComponent {
         } catch (Exception e) {
             throw new MapperParsingException("failed to parse mapping definition", e);
         }
-
-        if (root.keySet().size() == 0) {
-            throw new MapperParsingException("malformed mapping definition: no JSON root object found");
-        }
-
+        int size = root.size();
+        switch (size) {
+        case 0:
+            // if we don't have any keys throw an exception
+            throw new MapperParsingException("malformed mapping no root object found");
+        case 1:
+            break;
+        default:
             // we always assume the first and single key is the mapping type root
-        if (root.keySet().size() != 1) {
             throw new MapperParsingException("mapping must have the `type` as the root object");
+
         }
 
         String rootName = root.keySet().iterator().next();

src/test/java/org/elasticsearch/test/unit/index/mapper/source/DefaultSourceMappingTests.java

@@ -28,18 +28,18 @@ import org.elasticsearch.index.mapper.DocumentMapper;
 import org.elasticsearch.index.mapper.MapperParsingException;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.mapper.ParsedDocument;
+import org.elasticsearch.test.integration.ElasticsearchTestCase;
 import org.elasticsearch.test.unit.index.mapper.MapperTestUtils;
 import org.junit.Test;
 
 import java.util.Map;
 
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
 
 /**
  *
  */
-public class DefaultSourceMappingTests {
+public class DefaultSourceMappingTests extends ElasticsearchTestCase {
 
     @Test
     public void testNoFormat() throws Exception {
@@ -136,15 +136,23 @@ public class DefaultSourceMappingTests {
         DocumentMapper mapper = MapperTestUtils.newParser().parse("my_type", null, defaultMapping);
         assertThat(mapper.type(), equalTo("my_type"));
         assertThat(mapper.sourceMapper().enabled(), equalTo(false));
-
         try {
-            mapper = MapperTestUtils.newParser().parse(null, null, defaultMapping);
+            mapper = MapperTestUtils.newParser().parse(null, getRandom().nextBoolean() ? null : "", defaultMapping);
             assertThat(mapper.type(), equalTo("my_type"));
             assertThat(mapper.sourceMapper().enabled(), equalTo(false));
             assert false;
         } catch (MapperParsingException e) {
             // all is well
         }
+        try {
+            mapper = MapperTestUtils.newParser().parse(null, "{}", defaultMapping);
+            assertThat(mapper.type(), equalTo("my_type"));
+            assertThat(mapper.sourceMapper().enabled(), equalTo(false));
+            assert false;
+        } catch (MapperParsingException e) {
+            assertThat(e.getMessage(), equalTo("malformed mapping no root object found"));
+            // all is well
+        }
     }
     
     @Test