fix(common): don't convert null to a string when flushing a mock request (#21417)

A bug in TestRequest caused null response bodies to be stringified. This
change causes null to be treated faithfully.

Fixes #20744

PR Close #21417

packages/common/http/testing/BUILD.bazel

@@ -5,7 +5,12 @@ load("//tools:defaults.bzl", "ts_library")
 ts_library(
     name = "testing",
     testonly = 1,
-    srcs = glob(["**/*.ts"]),
+    srcs = glob(
+        [
+            "*.ts",
+            "src/**/*.ts",
+        ],
+    ),
     module_name = "@angular/common/http/testing",
     deps = [
         "//packages/common/http",