Issue lampepfl#7113: Fix Scala.js codegen for Null/Nothing types

- Fix the encoding of the `NullClass` and `NothingClass` in `JSEncoding` to
  correctly map to the corresponding Scala.js IR Symbols
- Enable the `javalib/net/URITest` for Scala.js, which uses default
  arguments with `null` values, which in turn are used as the return
  type of the default getters.

Author: Lacaranian

compiler/src/dotty/tools/backend/sjs/JSEncoding.scala

@@ -207,6 +207,10 @@ object JSEncoding {
        * BoxedUnit$ is a JVM artifact.
        */
       ir.Definitions.BoxedUnitClass
+    } else if(sym == defn.NullClass) {
+      ir.Definitions.NullClass
+    } else if(sym == defn.NothingClass) {
+      ir.Definitions.NothingClass
     } else {
       ir.Definitions.encodeClassName(fullyMangledString(sym.fullName))
     }

project/Build.scala

@@ -981,7 +981,7 @@ object Build {
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/lang" ** (("*.scala": FileFilter) -- "ClassTest.scala" -- "StringTest.scala")).get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/io" ** (("*.scala": FileFilter) -- "ByteArrayInputStreamTest.scala" -- "ByteArrayOutputStreamTest.scala" -- "DataInputStreamTest.scala" -- "DataOutputStreamTest.scala" -- "InputStreamTest.scala" -- "OutputStreamWriterTest.scala" -- "PrintStreamTest.scala" -- "ReadersTest.scala" -- "CommonStreamsTests.scala")).get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/math" ** "*.scala").get
-          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/net" ** (("*.scala": FileFilter) -- "URITest.scala")).get
+          ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/net" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/security" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/util/regex" ** "*.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/javalib/util/concurrent" ** (("*.scala": FileFilter) -- "ConcurrentHashMapTest.scala" -- "ConcurrentLinkedQueueTest.scala" -- "ConcurrentMapTest.scala" -- "ConcurrentSkipListSetTest.scala" -- "CopyOnWriteArrayListTest.scala")).get