Fix ExecuteAndWait with empty environment on Windows (llvm#158719)

CreateProcessW requires that the environemnt block to be always double
null-terminated even with an empty environemnt.

https://learn.microsoft.com/en-us/windows/win32/procthread/environment-variables

The attached test fails this way without the fix.

C:\Users\hiroshi\upstream\llvm-project\llvm\unittests\Support\ProgramTest.cpp(697):
error: Value of: ExecutionFailed
  Actual: true
  Expected: false
Couldn't execute program
'C:\Users\hiroshi\upstream\llvm-project\build\unittests\Support\SupportTests.exe':
The parameter is incorrect. (0x57)

(Cherry picked from commit 81c55e6)

Author: hjyamauchi

llvm/lib/Support/Windows/Program.inc

@@ -220,6 +220,10 @@ static bool Execute(ProcessInfo &PI, StringRef Program,
       llvm::append_range(EnvBlock, EnvString);
       EnvBlock.push_back(0);
     }
+    // If an empty environment (*Env is size zero), we need to
+    // terminate with two nulls.
+    if (Env->size() == 0)
+      EnvBlock.push_back(0);
     EnvBlock.push_back(0);
   }
 

llvm/unittests/Support/ProgramTest.cpp

@@ -597,4 +597,22 @@ TEST_F(ProgramEnvTest, TestExecuteWithNoStacktraceHandler) {
   ASSERT_EQ(0, RetCode);
 }
 
+TEST_F(ProgramEnvTest, TestExecuteEmptyEnvironment) {
+  using namespace llvm::sys;
+
+  std::string Executable =
+      sys::fs::getMainExecutable(TestMainArgv0, &ProgramTestStringArg1);
+  StringRef argv[] = {
+      Executable,
+      "--gtest_filter=" // A null invocation to avoid infinite recursion
+  };
+
+  std::string Error;
+  bool ExecutionFailed;
+  int RetCode = ExecuteAndWait(Executable, argv, ArrayRef<StringRef>{}, {}, 0,
+                               0, &Error, &ExecutionFailed);
+  EXPECT_FALSE(ExecutionFailed) << Error;
+  ASSERT_EQ(0, RetCode);
+}
+
 } // end anonymous namespace