Add repo env test (bazelbuild#15768)

Fix and test for null value bug when using --repo_env
[Issue Link](bazelbuild#15430)

Closes bazelbuild#15618.

PiperOrigin-RevId: 457728307
Change-Id: I3d57ae4ac95de09215281bec51329b7baafd53af

Co-authored-by: Googler <noreply@google.com>

src/main/java/com/google/devtools/build/lib/runtime/CommandEnvironment.java

@@ -262,8 +262,8 @@ public void exit(AbruptExitException exception) {
         value = System.getenv(name);
       }
       if (value != null) {
-        repoEnv.put(entry.getKey(), entry.getValue());
-        repoEnvFromOptions.put(entry.getKey(), entry.getValue());
+        repoEnv.put(name, value);
+        repoEnvFromOptions.put(name, value);
       }
     }
   }

src/test/py/bazel/bazel_external_repository_test.py

@@ -427,5 +427,27 @@ def testExternalBazelignoreContainingRepoName(self):
         args=['build', '@other_repo//:file'], cwd=work_dir)
     self.AssertExitCode(exit_code, 0, stderr)
 
+  def testRepoEnv(self):
+    # Testing fix for issue: https://github.com/bazelbuild/bazel/issues/15430
+
+    self.ScratchFile('WORKSPACE', [
+        'load("//:main.bzl", "dump_env")', 'dump_env(', '    name = "debug"',
+        ')'
+    ])
+    self.ScratchFile('BUILD')
+    self.ScratchFile('main.bzl', [
+        'def _dump_env(ctx):', '    val = ctx.os.environ.get("FOO", "nothing")',
+        '    print("FOO", val)', '    ctx.file("BUILD")',
+        'dump_env = repository_rule(', '    implementation = _dump_env,',
+        '    local = True,', ')'
+    ])
+
+    exit_code, _, stderr = self.RunBazel(
+        args=['build', '@debug//:all', '--repo_env=FOO'],
+        env_add={'FOO': 'bar'})
+    self.AssertExitCode(exit_code, 0, stderr)
+    self.assertIn('FOO bar', os.linesep.join(stderr))
+    self.assertNotIn('null value in entry: FOO=null', os.linesep.join(stderr))
+
 if __name__ == '__main__':
   unittest.main()