throw an error instead of a panic if no input is provided to inspect (#9259)

# Description

This is a small PR to fix Nu crashing when calling `inspect` with no
data piped in(#9255).


# User-Facing Changes

none.

crates/nu-command/tests/commands/inspect.rs

@@ -0,0 +1,7 @@
+use nu_test_support::nu;
+
+#[test]
+fn inspect_with_empty_pipeline() {
+    let actual = nu!("inspect");
+    assert!(actual.err.contains("no input value was piped in"));
+}

crates/nu-command/tests/commands/mod.rs

@@ -37,6 +37,7 @@ mod headers;
 mod help;
 mod histogram;
 mod insert;
+mod inspect;
 mod into_filesize;
 mod into_int;
 mod join;