Fix dedent tokens being nonempty

When the dedent had some whitespace after it (not an empty line),
that whitespace was considered part of the dedent token,
causing the CST node range to be off

packages/langium/src/parser/indentation-aware.ts

@@ -317,14 +317,14 @@ export class IndentationAwareTokenBuilder<Terminals extends string = string, Key
             const token = this.createIndentationTokenInstance(
                 this.dedentTokenType,
                 text,
-                match?.[0] ?? '',
+                '',  // Dedents are 0-width tokens
                 offset,
             );
             tokens.push(token);
             this.indentationStack.pop();
         }
 
-        // Token already added, let the dedentation now be consumed as whitespace and ignored
+        // Token already added, let the dedentation now be consumed as whitespace (if any) and ignored
         return null;
     }
 
@@ -356,7 +356,7 @@ export class IndentationAwareTokenBuilder<Terminals extends string = string, Key
         const remainingDedents: IToken[] = [];
         while (this.indentationStack.length > 1) {
             remainingDedents.push(
-                this.createIndentationTokenInstance(this.dedentTokenType, text, this.options.dedentTokenName, text.length)
+                this.createIndentationTokenInstance(this.dedentTokenType, text, '', text.length)
             );
             this.indentationStack.pop();
         }