Fix DCHECK crash.

Previously, we failed this DCHECK:
https://cs.chromium.org/chromium/src/ui/views/controls/styled_label.cc?dr=CSs&q=styled_label&g=0&l=156

This occurred when we had an empty query string.

Bug: b:112773506
Change-Id: I12f2418a7c0a058fe52ffff6c3507c844c607276
Reviewed-on: https://chromium-review.googlesource.com/1180523
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584278}

ash/assistant/ui/main_stage/assistant_query_view.cc

@@ -98,32 +98,31 @@ void AssistantQueryView::SetQuery(const AssistantQuery& query) {
 
 void AssistantQueryView::SetText(const std::string& high_confidence_text,
                                  const std::string& low_confidence_text) {
-  // High confidence text and low confidence text are displayed in different
-  // colors for visual emphasis.
-  const base::string16& high_confidence_text_16 =
-      base::UTF8ToUTF16(high_confidence_text);
-
-  if (low_confidence_text.empty()) {
-    label_->SetText(high_confidence_text_16);
-    label_->AddStyleRange(gfx::Range(0, high_confidence_text_16.length()),
-                          CreateStyleInfo(kTextColorPrimary));
+  if (high_confidence_text.empty() && low_confidence_text.empty()) {
+    label_->SetText(base::string16());
   } else {
+    const base::string16& high_confidence_text_16 =
+        base::UTF8ToUTF16(high_confidence_text);
+
     const base::string16& low_confidence_text_16 =
         base::UTF8ToUTF16(low_confidence_text);
 
     label_->SetText(high_confidence_text_16 + low_confidence_text_16);
 
-    // High confidence text styling.
-    label_->AddStyleRange(gfx::Range(0, high_confidence_text_16.length()),
-                          CreateStyleInfo(kTextColorPrimary));
+    // Style high confidence text.
+    if (!high_confidence_text_16.empty()) {
+      label_->AddStyleRange(gfx::Range(0, high_confidence_text_16.length()),
+                            CreateStyleInfo(kTextColorPrimary));
+    }
 
-    // Low confidence text styling.
-    label_->AddStyleRange(gfx::Range(high_confidence_text_16.length(),
-                                     high_confidence_text_16.length() +
-                                         low_confidence_text_16.length()),
-                          CreateStyleInfo(kTextColorHint));
+    // Style low confidence text.
+    if (!low_confidence_text_16.empty()) {
+      label_->AddStyleRange(gfx::Range(high_confidence_text_16.length(),
+                                       high_confidence_text_16.length() +
+                                           low_confidence_text_16.length()),
+                            CreateStyleInfo(kTextColorHint));
+    }
   }
-
   label_->SizeToFit(width());
   PreferredSizeChanged();
 }