Fix issues with null safety

Author: tgodzik

compiler/src/scala/quoted/runtime/impl/printers/SourceCode.scala

@@ -1451,8 +1451,8 @@ object SourceCode {
     private def escapedChar(ch: Char): String =
       if requiresFormat(ch) then
         val b = StringBuilder().append(qc)
-        escapedChar(b, ch)
-        b.append(qc).toString
+        escapedChar(b.nn, ch)
+        b.nn.append(qc).toString
       else
         qc + ch + qc