funding: use default fwding policy if persisted values not found

In this commit, a bug is fixed in the funding manager that could result
in the funding process erroring out if the persisted initial forwarding
policy is not found. This could occur if a node restarts after opening a
channel that is not yet fully confirmed and also upgrades their node
from a pre-0.16 version to 0.16 since the values are only expected to be
persisted after 0.16.

docs/release-notes/release-notes-0.16.1.md

@@ -84,11 +84,14 @@ available](https://github.com/lightningnetwork/lnd/pull/7529).
   
 * [Fix log output](https://github.com/lightningnetwork/lnd/pull/7604).
 
-
 * [Channels opened with custom fee policies are now able to forward payments
   correctly without needing to restart
   first](https://github.com/lightningnetwork/lnd/pull/7597).
 
+* [Fix a bug](https://github.com/lightningnetwork/lnd/pull/7613) where the 
+  funding manager would error out if no persisted initial forwarding policy is 
+  found for a channel. 
+
 # Contributors (Alphabetical Order)
 
 * ardevd