[dagit] Fix CodeMirror show-hint error (#7334)

## Summary

There is an error within the CodeMirror show-hint addon that fires on an empty Launchpad. This appears to be because we aren't providing `from` and `to` values in the return value for the `hint` helper function, and they are expected to be there. Add them.

## Test Plan

View Launchpad, clear out editor config. Start typing in the empty editor, verify that hint suggestions render properly and that there are no JS errors in the console.

js_modules/dagit/packages/core/src/configeditor/codemirror-yaml/mode.tsx

@@ -308,11 +308,7 @@ CodeMirror.registerHelper(
     options: {
       schema?: ConfigEditorRunConfigSchemaFragment;
     },
-  ): {list: Array<CodemirrorHint>} => {
-    if (!options.schema) {
-      return {list: []};
-    }
-
+  ): {list: Array<CodemirrorHint>; from: CodemirrorLocation; to: CodemirrorLocation} => {
     const {
       cursor,
       context,
@@ -321,8 +317,16 @@ CodeMirror.registerHelper(
       searchString,
       prevToken,
     } = expandAutocompletionContextAtCursor(editor);
+
+    const from = {line: cursor.line, ch: start};
+    const to = {line: cursor.line, ch: token.end};
+
+    if (!options.schema) {
+      return {list: [], from, to};
+    }
+
     if (!context) {
-      return {list: []};
+      return {list: [], from, to};
     }
 
     // Since writing meaningful tests for this functionality is difficult given a) no jsdom
@@ -433,8 +437,8 @@ CodeMirror.registerHelper(
         }
         el.appendChild(div);
       },
-      from: {line: cursor.line, ch: start},
-      to: {line: cursor.line, ch: token.end},
+      from,
+      to,
     });
 
     // Calculate if this is on a new-line child of a scalar union type, as an indication that we
@@ -458,6 +462,8 @@ CodeMirror.registerHelper(
               field.description,
             ),
           ),
+        from,
+        to,
       };
     }
 
@@ -470,6 +476,8 @@ CodeMirror.registerHelper(
         list: context.type.values
           .filter((val) => val.value.startsWith(searchWithoutQuotes))
           .map((val) => buildSuggestion(val.value, `"${val.value}"`, null)),
+        from,
+        to,
       };
     }
 
@@ -479,6 +487,8 @@ CodeMirror.registerHelper(
         list: ['True', 'False']
           .filter((val) => val.startsWith(searchString))
           .map((val) => buildSuggestion(val, val, null)),
+        from,
+        to,
       };
     }
 
@@ -513,10 +523,10 @@ CodeMirror.registerHelper(
           );
       }
 
-      return {list: [...scalarSuggestions, ...nonScalarSuggestions]};
+      return {list: [...scalarSuggestions, ...nonScalarSuggestions], from, to};
     }
 
-    return {list: []};
+    return {list: [], from, to};
   },
 );