check if bootstrap buildroot has tpmdir

Fixes #1351

mock/py/mockbuild/buildroot.py

@@ -880,7 +880,7 @@ def _setup_nosync(self):
         multilib = ('x86_64', 's390x')
         # ld_preload need to be same as in bootstrap because we call DNF in bootstrap, but
         # but it will load nosync from the final chroot
-        if self.bootstrap_buildroot is not None:
+        if self.bootstrap_buildroot is not None and self.bootstrap_buildroot.tmpdir is not None:
             self.tmpdir = self.bootstrap_buildroot.tmpdir
             if not os.path.isdir(self.tmpdir):
                 os.mkdir(self.tmpdir, 0o700)