Enable conditional source and use patterns by allowing null as a no-op module (#14773)

Related:
- #14329
- #13872
- #8214

# Description & User-Facing Changes

This PR allows enables the following uses, which are all no-op.
```nushell
source null
source-env null
use null
overlay use null
```

The motivation for this change is conditional sourcing of files. For
example, with this change `login.nu` may be deprecated and replaced with
the following code in `config.nu`
```nushell
const login_module = if $nu.is-login { "login.nu" } else { null }
source $login_module
```

# Tests + Formatting
I'm hoping for CI to pass 😄

# After Submitting
Add a part about the conditional sourcing pattern to the website.

crates/nu-cmd-lang/src/core_commands/overlay/use_.rs

@@ -24,8 +24,8 @@ impl Command for OverlayUse {
             .allow_variants_without_examples(true)
             .required(
                 "name",
-                SyntaxShape::String,
-                "Module name to use overlay for.",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]),
+                "Module name to use overlay for (`null` for no-op).",
             )
             .optional(
                 "as",
@@ -61,6 +61,11 @@ impl Command for OverlayUse {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        let noop = call.get_parser_info(caller_stack, "noop");
+        if noop.is_some() {
+            return Ok(PipelineData::empty());
+        }
+
         let mut name_arg: Spanned<String> = call.req(engine_state, caller_stack, 0)?;
         name_arg.item = trim_quotes_str(&name_arg.item).to_string();
 

crates/nu-cmd-lang/src/core_commands/use_.rs

@@ -22,7 +22,11 @@ impl Command for Use {
         Signature::build("use")
             .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .allow_variants_without_examples(true)
-            .required("module", SyntaxShape::String, "Module or module file.")
+            .required(
+                "module",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]),
+                "Module or module file (`null` for no-op).",
+            )
             .rest(
                 "members",
                 SyntaxShape::Any,
@@ -54,6 +58,9 @@ This command is a parser keyword. For details, check:
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(caller_stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
         let Some(Expression {
             expr: Expr::ImportPattern(import_pattern),
             ..