Fix input redirect for externals (#7387)

# Description

Ooops, fix the input redirection logic so we don't break vim.

# User-Facing Changes

_(List of all changes that impact the user experience here. This helps
us keep track of breaking changes.)_

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/system/run_external.rs

@@ -521,7 +521,7 @@ impl ExternalCommand {
 
         // If there is an input from the pipeline. The stdin from the process
         // is piped so it can be used to send the input information
-        if !matches!(input, PipelineData::Value(Value::Nothing { .. }, ..)) {
+        if !input.is_nothing() {
             process.stdin(Stdio::piped());
         }