wallet/db: make IsDummy() const

This method does a simple check and no modifications.

src/wallet/db.h

@@ -193,7 +193,7 @@ private:
      * Only to be used at a low level, application should ideally not care
      * about this.
      */
-    bool IsDummy() { return env == nullptr; }
+    bool IsDummy() const { return env == nullptr; }
 };
 
 /** RAII class that provides access to a Berkeley database */