sw content controls: only try to insert placeholders if there is no selection

The logic is that in case there is no selection, we insert a placeholder
like "type here", but if there is a selection, that can be a custom
placeholder.

Without this fix, we would hit an assertion failure on inserting content
control when there is a selection, as the placeholder is empty and
operator[](0) is not allowed for empty OUStrings.

Change-Id: I8370b8d74415faf0e76dccb574aa29bea1947bb8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/134041
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins

sw/source/uibase/wrtsh/wrtsh1.cxx

@@ -1050,9 +1050,12 @@ void SwWrtShell::InsertContentControl(SwContentControlType eType)
             break;
         }
     }
-    Insert(aPlaceholder);
-    Left(CRSR_SKIP_CHARS, /*bSelect=*/true, aPlaceholder.getLength(),
-            /*bBasicCall=*/false);
+    if (aPlaceholder.getLength())
+    {
+        Insert(aPlaceholder);
+        Left(CRSR_SKIP_CHARS, /*bSelect=*/true, aPlaceholder.getLength(),
+                /*bBasicCall=*/false);
+    }
     SwFormatContentControl aContentControl(pContentControl, RES_TXTATR_CONTENTCONTROL);
     SetAttrItem(aContentControl);
 }