db/DB.js: prevent DB layer from returning undefined

ueberDB2 can return either undefined or null for a missing key, depending on
which DB driver is used. This patch changes the promise version of the API so
that it will always return null.

src/node/db/DB.js

@@ -51,6 +51,19 @@ exports.init = function() {
           exports[fn] = util.promisify(db[fn].bind(db));
         });
 
+        // set up wrappers for get and getSub that can't return "undefined"
+        let get = exports.get;
+        exports.get = async function(key) {
+          let result = await get(key);
+          return (result === undefined) ? null : result;
+        };
+
+        let getSub = exports.getSub;
+        exports.getSub = async function(key, sub) {
+          let result = await getSub(key, sub);
+          return (result === undefined) ? null : result;
+        };
+
         // exposed for those callers that need the underlying raw API
         exports.db = db;
         resolve();