Ignore empty completion input (#30713)

This change makes sure that an empty completion input does not throw an IAE when indexing.
Instead the input is ignored and the completion field is added in the list of ignored fields
for the document.

Closes #23121

Author: jimczi

server/src/main/java/org/elasticsearch/index/mapper/CompletionFieldMapper.java

@@ -449,6 +449,10 @@ public Mapper parse(ParseContext context) throws IOException {
         // index
         for (Map.Entry<String, CompletionInputMetaData> completionInput : inputMap.entrySet()) {
             String input = completionInput.getKey();
+            if (input.trim().isEmpty()) {
+                context.addIgnoredField(fieldType.name());
+                continue;
+            }
             // truncate input
             if (input.length() > maxInputLength) {
                 int len = Math.min(maxInputLength, input.length());

server/src/test/java/org/elasticsearch/index/mapper/CompletionFieldMapperTests.java

@@ -397,6 +397,19 @@ public void testFieldValueValidation() throws Exception {
             assertThat(cause, instanceOf(IllegalArgumentException.class));
             assertThat(cause.getMessage(), containsString("[0x1e]"));
         }
+
+        // empty inputs are ignored
+        ParsedDocument doc = defaultMapper.parse(SourceToParse.source("test", "type1", "1", BytesReference
+                .bytes(XContentFactory.jsonBuilder()
+                    .startObject()
+                    .array("completion", "   ", "")
+                    .endObject()),
+            XContentType.JSON));
+        assertThat(doc.docs().size(), equalTo(1));
+        assertNull(doc.docs().get(0).get("completion"));
+        assertNotNull(doc.docs().get(0).getField("_ignored"));
+        IndexableField ignoredFields = doc.docs().get(0).getField("_ignored");
+        assertThat(ignoredFields.stringValue(), equalTo("completion"));
     }
 
     public void testPrefixQueryType() throws Exception {