Add existence test to l3prop functions as a temporary fix

This is meant as a temporary measure to avoid breaking packages.
I'm not totally sure if it covers the right set of functions.
More testing welcome.  I think this should be reverted in a
couple of weeks after the freeze.

l3kernel/l3prop.dtx

@@ -1014,9 +1014,18 @@
 %   make the second choice; this way props that are accidentally
 %   \tn{relax} are treated as they were before.  The auxiliary receives
 %   \cs{use_i:nn} or \cs{use_ii:nn} as~|#3|.
+%   The existence test (and its arguments) are just there to temporarily
+%   avoid breaking some packages before the TeXlive freeze, and should
+%   be removed in later revisions.  We use a global assignment like in
+%   \cs{prop_new:N}.
 %    \begin{macrocode}
 \cs_new:Npn \@@_if_flat:NTF #1
   {
+    \cs_if_exist:NF #1
+      {
+        \cs_gset_eq:NN #1 \c_empty_prop
+        \@@_if_flat_aux:w \@@_flatten:w \s_@@_mark \use_i:nn
+      }
     \exp_after:wN \@@_if_flat_aux:w #1
     \s_@@_mark \use_ii:nn
     \@@_flatten:w \s_@@_mark \use_i:nn \s_@@_stop