win: support retrieving empty env variables

This commit adds Windows support for retrieving empty
environment variables via uv_os_getenv().

src/win/util.c

@@ -1481,6 +1481,7 @@ int uv_os_getenv(const char* name, char* buffer, size_t* size) {
   if (r != 0)
     return r;
 
+  SetLastError(ERROR_SUCCESS);
   len = GetEnvironmentVariableW(name_w, var, MAX_ENV_VAR_LENGTH);
   uv__free(name_w);
   assert(len < MAX_ENV_VAR_LENGTH); /* len does not include the null */
@@ -1491,7 +1492,9 @@ int uv_os_getenv(const char* name, char* buffer, size_t* size) {
     if (r == ERROR_ENVVAR_NOT_FOUND)
       return UV_ENOENT;
 
-    return uv_translate_sys_error(r);
+    /* Handle defined, but empty environment variables. */
+    if (r != ERROR_SUCCESS)
+      return uv_translate_sys_error(r);
   }
 
   /* Check how much space we need */

test/test-env-vars.c

@@ -88,6 +88,15 @@ TEST_IMPL(env_vars) {
   r = uv_os_unsetenv(name);
   ASSERT(r == 0);
 
+  /* Setting an environment variable to the empty string does not delete it. */
+  r = uv_os_setenv(name, "");
+  ASSERT(r == 0);
+  size = BUF_SIZE;
+  r = uv_os_getenv(name, buf, &size);
+  ASSERT(r == 0);
+  ASSERT(size == 0);
+  ASSERT(strlen(buf) == 0);
+
   /* Check getting all env variables. */
   r = uv_os_setenv(name, "123456789");
   ASSERT(r == 0);