Send empty string instead of undefined for text property

GrTextarea has "text" property as string hence don't pass
undefined.

Google-bug-id: b/247077317
Release-Notes: skip
Change-Id: I5266824b5078fce8076f12cc376a121099c77e8f
diff --git a/polygerrit-ui/app/elements/admin/gr-group/gr-group.ts b/polygerrit-ui/app/elements/admin/gr-group/gr-group.ts
index 681eb3ae..e65b16b 100644
--- a/polygerrit-ui/app/elements/admin/gr-group/gr-group.ts
+++ b/polygerrit-ui/app/elements/admin/gr-group/gr-group.ts
@@ -230,7 +230,7 @@
             rows="4"
             monospace
             ?disabled=${this.computeGroupDisabled()}
-            .text=${this.groupConfig?.description}
+            .text=${this.groupConfig?.description ?? ''}
             @text-changed=${this.handleDescriptionTextChanged}
           ></gr-textarea>
         </div>
diff --git a/polygerrit-ui/app/elements/admin/gr-repo/gr-repo.ts b/polygerrit-ui/app/elements/admin/gr-repo/gr-repo.ts
index 8e69806..01d452a 100644
--- a/polygerrit-ui/app/elements/admin/gr-repo/gr-repo.ts
+++ b/polygerrit-ui/app/elements/admin/gr-repo/gr-repo.ts
@@ -261,7 +261,7 @@
           rows="4"
           monospace
           ?disabled=${this.readOnly}
-          .text=${this.repoConfig?.description}
+          .text=${this.repoConfig?.description ?? ''}
           @text-changed=${this.handleDescriptionTextChanged}
         ></gr-textarea>
       </fieldset>