Add null as dummy TestMessage parameter

Fixes the `test_demonstrate_INCORRECT_scala_proto_library_stamp` test
case from `test/shell/test_strict_dependency.sh` after the ScalaPB
0.11.17 bump.

Somehow, bumping that library (presumably, instead of gRPC, et. al.)
changed the parameter list of `TestMessage` from `String` to
`String,UnknownFieldSet`. The test then failed because the following
command produced this unexpected error:

```txt
$ bazel build --verbose_failures \
  //test_expect_failure/missing_direct_deps/scala_proto_deps:uses_transitive_scala_proto \
  --extra_toolchains=//test/toolchains:ast_plus_one_deps_strict_deps_error

ERROR: .../test_expect_failure/missing_direct_deps/scala_proto_deps/BUILD:25:13:
  Building test_expect_failure/missing_direct_deps/scala_proto_deps/libtransitive.jar
  (1 source file) failed: (Exit 1): java failed: error executing command
  (from target //test_expect_failure/missing_direct_deps/scala_proto_deps:transitive)

  (cd ...
    [ ...snip... ]
  external/remotejdk11_macos_aarch64/bin/java -XX:-CompactStrings
    [ ...snip... ]
  external/remote_java_tools/java_tools/JavaBuilder_deploy.jar
  @bazel-out/.../bin/test_expect_failure/missing_direct_deps/scala_proto_deps/libtransitive.jar-0.params
  @bazel-out/.../bin/test_expect_failure/missing_direct_deps/scala_proto_deps/libtransitive.jar-1.params)

test_expect_failure/missing_direct_deps/scala_proto_deps/UseTestMessage.java:7:
  error: constructor TestMessage in class TestMessage
  cannot be applied to given types;

  private final TestMessage m = new TestMessage("");
                                ^
  required: String,UnknownFieldSet
  found: String
  reason: actual and formal argument lists differ in length
```

After this change, the same command produces the following failure that
the test is expecting to catch:

```txt
ERROR: .../test_expect_failure/missing_direct_deps/scala_proto_deps/BUILD:18:14:
  scala @//test_expect_failure/missing_direct_deps/scala_proto_deps:uses_transitive_scala_proto
  failed: (Exit 1): scalac failed: error executing command
  (from target //test_expect_failure/missing_direct_deps/scala_proto_deps:uses_transitive_scala_proto)

  (cd .../execroot/io_bazel_rules_scala && \
  exec env - \
  bazel-out/.../bin/src/java/io/bazel/rulesscala/scalac/scalac
  @bazel-out/.../bin/test_expect_failure/missing_direct_deps/scala_proto_deps/uses_transitive_scala_proto.jar-0.params)

test_expect_failure/missing_direct_deps/scala_proto_deps/UseScalaProtoIndirectly.scala:6:
  error: Target '@//test_expect_failure/missing_direct_deps/scala_proto_deps:some_proto'
  is used but isn't explicitly declared, please add it to the deps.

You can use the following buildozer command:
  buildozer 'add deps
    @//test_expect_failure/missing_direct_deps/scala_proto_deps:some_proto'
    //test_expect_failure/missing_direct_deps/scala_proto_deps:uses_transitive_scala_proto

  val foo: TestMessage = new UseTestMessage().getM
           ^
one error found
Build failure with errors.
Target //test_expect_failure/missing_direct_deps/scala_proto_deps:uses_transitive_scala_proto
  failed to build
```

Author: mbland

test_expect_failure/missing_direct_deps/scala_proto_deps/UseTestMessage.java

@@ -4,7 +4,7 @@
 
 public class UseTestMessage {
 
-  private final TestMessage m = new TestMessage("");
+  private final TestMessage m = new TestMessage("", null);
 
   public TestMessage getM() {
     return m;