Do not pass non-string env vars to externals (#4748)

* Do not pass non-string env vars to externals

Also misc cleanup

* Add note to default config

* Add a test

* Ensure PATH/Path conversion list <-> string

src/tests/test_env.rs

@@ -14,3 +14,11 @@ fn shorthand_env_2() -> TestResult {
 fn shorthand_env_3() -> TestResult {
     run_test(r#"FOO=BAZ BAR=MOO $env.FOO"#, "BAZ")
 }
+
+#[test]
+fn convert_non_string_env_var_to_nothing() -> TestResult {
+    run_test(
+        r#"let-env FOO = true; env | where name == FOO | get raw.0 | describe"#,
+        "nothing",
+    )
+}