fix: 패널 해시태그 null 처리 문제 해결

Author: hardwoong

src/main/java/DiffLens/back_end/domain/panel/service/PanelService.java

@@ -143,7 +143,7 @@ private List<PanelResponseDTO.PanelDetails.PanelDetail.Attribute> createAttribut
                                                 .build(),
                                 PanelResponseDTO.PanelDetails.PanelDetail.Attribute.builder()
                                                 .key("hash_tags")
-                                                .value(panel.getHashtags().toString())
+                                                .value(panel.getHashtags() != null ? panel.getHashtags().toString() : null)
                                                 .build(),
                                 PanelResponseDTO.PanelDetails.PanelDetail.Attribute.builder()
                                                 .key("raw_data")