Fixed the panic when type a statement similar to let f = 'f' $ in the nushell (#9851)

- this PR should close #9596 
- fixes #9596 
- this PR should close #9826 
- fixes #9826 

fixed the following bugs:
```nu
# type following statements in the nushell
let f = 'f' $;
mut f = 'f' $;
const f = 'f' $;

# then remove variable f, it will panics
let = 'f' $;
mut  = 'f' $;
const = 'f' $;
```

src/tests/test_parser.rs

@@ -145,6 +145,25 @@ fn bad_var_name2() -> TestResult {
     fail_test(r#"let $foo-bar = 4"#, "valid variable")
 }
 
+#[test]
+fn assignment_with_no_var() -> TestResult {
+    let cases = [
+        "let = if $",
+        "mut = if $",
+        "const = if $",
+        "let = 'foo' | $in; $x | describe",
+        "mut = 'foo' | $in; $x | describe",
+    ];
+
+    let expected = "valid variable";
+
+    for case in cases {
+        fail_test(case, expected)?;
+    }
+
+    Ok(())
+}
+
 #[test]
 fn long_flag() -> TestResult {
     run_test(