Fix NPE in SemanticTextHighlighter

If the model settings is null then the field is guaranteed to not have any indexed content.

Closes #129501

Author: jimczi

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/highlight/SemanticTextHighlighter.java

@@ -74,7 +74,7 @@ public HighlightField highlight(FieldHighlightContext fieldContext) throws IOExc
             return null;
         }
         SemanticTextFieldType fieldType = (SemanticTextFieldType) fieldContext.fieldType;
-        if (fieldType.getEmbeddingsField() == null) {
+        if (fieldType.getModelSettings() == null || fieldType.getEmbeddingsField() == null) {
             // nothing indexed yet
             return null;
         }

x-pack/plugin/inference/src/yamlRestTest/resources/rest-api-spec/test/inference/90_semantic_text_highlighter.yml

@@ -60,6 +60,9 @@ setup:
               body:
                 type: semantic_text
                 inference_id: dense-inference-id
+              another_body:
+                type: semantic_text
+                inference_id: dense-inference-id
 
   - do:
       index:
@@ -79,6 +82,22 @@ setup:
           body: [ "ElasticSearch is an open source, distributed, RESTful, search engine which is built on top of Lucene internally and enjoys all the features it provides.", "You Know, for Search!" ]
         refresh: true
 
+---
+"Highlighting empty index":
+  - do:
+      search:
+        index: test-sparse-index
+        body:
+          query:
+            match_all: {}
+          highlight:
+            fields:
+              another_body: {}
+
+  - match:      { hits.total.value: 1 }
+  - match:      { hits.hits.0._id: "doc_1" }
+  - not_exists:   hits.hits.0.highlight.another_body
+
 ---
 "Highlighting using a sparse embedding model":
   - do: