Ignore empty string on language detection

Relates #6

Author: spinscale

src/main/java/org/elasticsearch/plugin/ingest/langdetect/LangDetectProcessor.java

@@ -19,6 +19,7 @@
 
 import com.cybozu.labs.langdetect.Detector;
 import com.cybozu.labs.langdetect.DetectorFactory;
+import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.common.unit.ByteSizeValue;
 import org.elasticsearch.ingest.AbstractProcessor;
@@ -64,6 +65,10 @@ public IngestDocument execute(IngestDocument ingestDocument) throws Exception {
             }
             throw e;
         }
+        if (Strings.isEmpty(content)) {
+            return ingestDocument;
+        }
+
         detector.append(content);
         String language = detector.detect();
 

src/test/java/org/elasticsearch/plugin/ingest/langdetect/LangDetectProcessorTests.java

@@ -86,6 +86,18 @@ public void testIgnoreMissingConfiguration() throws Exception {
         assertThat(data, not(hasEntry("language", "en")));
     }
 
+    public void testEmptyString() throws Exception {
+        Map<String, Object> config = new HashMap<>();
+        config.put("field", "source_field");
+        config.put("target_field", "language");
+        config.put("ignore_missing", randomBoolean());
+
+        Map<String, Object> data = ingestDocument(config,"source_field", "");
+
+        assertThat(data, not(hasEntry("language", "en")));
+
+    }
+
     private Map<String, Object> ingestDocument(Map<String, Object> config, String field, String value) throws Exception {
         Map<String, Object> document = new HashMap<>();
         document.put(field, value);