Fix issue with joinFirstPair

Author: ruudk

src/CodeGenerator.php

@@ -477,7 +477,13 @@ public function joinFirstPair(array | Closure | Generator | string $data) : Gene
 
                 if ($line instanceof Group && $first !== null) {
                     $prefix = $first instanceof Group ? '' : (string) $first;
-                    yield Group::indent($this->prefixFirst($prefix, $line->lines), $line->indention);
+                    $lines = self::resolveIterable($line->lines);
+
+                    if ($lines === []) {
+                        yield $prefix;
+                    } else {
+                        yield Group::indent($this->prefixFirst($prefix, $lines), $line->indention);
+                    }
 
                     continue;
                 }

tests/CodeGeneratorTest.php

@@ -786,9 +786,8 @@ public function testJoinFirstPairWithEmptySecondGroup() : void
 
         $result = $this->generator->joinFirstPair($data);
 
-        // Note: When the second element is an empty group, 'first' gets lost
-        // This might be unexpected behavior
         $expected = <<<'PHP'
+            first
             third
             PHP;