Change ScriptPubKeyMan::Upgrade to default to return true

If a ScriptPubKeyMan does not implement Upgrade, then using upgraewallet
will fail unexpectedly. By changing the default to return true, then
this error can be avoided. This is still correct because a successful
upgrade can be that nothing happened.

Author: achow101

src/wallet/scriptpubkeyman.h

@@ -207,7 +207,7 @@ class ScriptPubKeyMan
     virtual bool CanGetAddresses(bool internal = false) const { return false; }
 
     /** Upgrades the wallet to the specified version */
-    virtual bool Upgrade(int prev_version, int new_version, bilingual_str& error) { return false; }
+    virtual bool Upgrade(int prev_version, int new_version, bilingual_str& error) { return true; }
 
     virtual bool HavePrivateKeys() const { return false; }