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-parser/src/parser.rs

@@ -15,7 +15,7 @@ use nu_engine::DIR_VAR_PARSER_INFO;
 use nu_protocol::{
     ast::*, engine::StateWorkingSet, eval_const::eval_constant, BlockId, DeclId, DidYouMean,
     FilesizeUnit, Flag, ParseError, PositionalArg, Signature, Span, Spanned, SyntaxShape, Type,
-    VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+    Value, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 use std::{
     collections::{HashMap, HashSet},
@@ -3054,6 +3054,14 @@ pub fn parse_import_pattern(working_set: &mut StateWorkingSet, spans: &[Span]) -
     let head_expr = parse_value(working_set, *head_span, &SyntaxShape::Any);
 
     let (maybe_module_id, head_name) = match eval_constant(working_set, &head_expr) {
+        Ok(Value::Nothing { .. }) => {
+            return Expression::new(
+                working_set,
+                Expr::Nothing,
+                Span::concat(spans),
+                Type::Nothing,
+            );
+        }
         Ok(val) => match val.coerce_into_string() {
             Ok(s) => (working_set.find_module(s.as_bytes()), s.into_bytes()),
             Err(err) => {