fix: issue #9 reserved keyword 'key' + issue #10 empty string treated as NULL

Issue 9: Add backtick-quoted identifier support in lexer.nim
- Users can now write `key` to use reserved keywords as column/table names

Issue 10: Use \N as internal NULL marker instead of empty string
- isNull() now checks for \N instead of value.len == 0
- Empty strings can now be stored in NOT NULL columns
- NULL values properly distinguished from empty strings in storage, wire protocol, and HTTP JSON responses

src/barabadb/core/httpserver.nim

@@ -164,9 +164,10 @@ proc queryHandler(server: HttpServer): RequestHandler =
           var jsonRow = newJObject()
           for col in res.columns:
             let key = col
-            var val = ""
-            if key in row: val = row[key]
-            jsonRow[key] = %val
+            if key in row and not isNull(row[key]):
+              jsonRow[key] = %row[key]
+            else:
+              jsonRow[key] = newJNull()
           jsonRows.add(jsonRow)
         var jsonCols = newJArray()
         for c in res.columns: