handle empty pipeline while parsing let (fix Issue10083) (#10116)

- fixes #10083 

# Description

nushell crashes in the following 2 condition:

- `let a = {}` , then delete `{`
- `let a = | {}`, then delete `{`

When delete `{` the pipeline becomes empty but current `nu-parser`
assume they are non-empty. This pr adds extra empty check to avoid
crash.


Co-authored-by: Horasal <horsal@horsal.dev>

crates/nu-parser/tests/test_parser.rs

@@ -1007,6 +1007,18 @@ mod string {
     }
 }
 
+#[rstest]
+#[case(b"let a = }")]
+#[case(b"mut a = }")]
+#[case(b"let a = | }")]
+#[case(b"mut a = | }")]
+fn test_semi_open_brace(#[case] phrase: &[u8]) {
+    let engine_state = EngineState::new();
+    let mut working_set = StateWorkingSet::new(&engine_state);
+    // this should not panic
+    let _block = parse(&mut working_set, None, phrase, true);
+}
+
 mod range {
     use super::*;
     use nu_protocol::ast::{RangeInclusion, RangeOperator};