Fix handling of empty corpus queries

Change-Id: Ia855430ded636287897cc00081d07b05c2fb37a8
diff --git a/Changes b/Changes
index dabde93..51adfa2 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,6 @@
-0.37 2020-10-23
+0.37 2020-11-19
     - [feature] Introduced query references in Poliqarp (diewald)
+    - [bugfix] Ignore empty corpus queries (diewald)
 
 0.36 2020-07-24
     - [security] Upgraded version of Google Guava
diff --git a/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java b/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
index c2e14e9..c74e155 100644
--- a/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
+++ b/src/main/java/de/ids_mannheim/korap/query/serialize/CollectionQueryProcessor.java
@@ -60,6 +60,9 @@
 
     @Override
     public void process (String query) {
+        if (query == null || query.length() == 0)
+            return;
+
         ParseTree tree = parseCollectionQuery(query);
         if (this.parser != null) {
             super.parser = this.parser;
diff --git a/src/test/java/de/ids_mannheim/korap/query/test/collection/CollectionQueryProcessorTest.java b/src/test/java/de/ids_mannheim/korap/query/test/collection/CollectionQueryProcessorTest.java
index 8f371de..5f97e40 100644
--- a/src/test/java/de/ids_mannheim/korap/query/test/collection/CollectionQueryProcessorTest.java
+++ b/src/test/java/de/ids_mannheim/korap/query/test/collection/CollectionQueryProcessorTest.java
@@ -95,6 +95,16 @@
 
 
     @Test
+    public void testEmpty () throws JsonProcessingException, IOException {
+        collection = "";
+        qs.setQuery(query, ql);
+        qs.setCollection(collection);
+        res = mapper.readTree(qs.toJSON());
+        assertFalse("Empty", res.has("/collection"));
+    }
+
+
+    @Test
     public void testSimple () throws JsonProcessingException, IOException {
         collection = "textClass=politik";
         qs.setQuery(query, ql);