fix: indent exist check before accessing its property (#328)

Author: pinal005

tomlkit/items.py

@@ -1412,6 +1412,7 @@ def __delitem__(self, key: int | slice):
                 if (
                     idx == 0
                     and len(self._value) > 0
+                    and self._value[idx].indent
                     and "\n" not in self._value[idx].indent.s
                 ):
                     # Remove the indentation of the first item if not newline