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-command/src/env/source_env.rs

@@ -19,8 +19,8 @@ impl Command for SourceEnv {
             .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "filename",
-                SyntaxShape::String, // type is string to avoid automatically canonicalizing the path
-                "The filepath to the script file to source the environment from.",
+                SyntaxShape::OneOf(vec![SyntaxShape::String, SyntaxShape::Nothing]), // type is string to avoid automatically canonicalizing the path
+                "The filepath to the script file to source the environment from (`null` for no-op).",
             )
             .category(Category::Core)
     }
@@ -45,6 +45,10 @@ impl Command for SourceEnv {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(caller_stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
+
         let source_filename: Spanned<String> = call.req(engine_state, caller_stack, 0)?;
 
         // Note: this hidden positional is the block_id that corresponded to the 0th position
@@ -99,10 +103,17 @@ impl Command for SourceEnv {
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![Example {
-            description: "Sources the environment from foo.nu in the current context",
-            example: r#"source-env foo.nu"#,
-            result: None,
-        }]
+        vec![
+            Example {
+                description: "Sources the environment from foo.nu in the current context",
+                example: r#"source-env foo.nu"#,
+                result: None,
+            },
+            Example {
+                description: "Sourcing `null` is a no-op.",
+                example: r#"source-env null"#,
+                result: None,
+            },
+        ]
     }
 }

crates/nu-command/src/misc/source.rs

@@ -16,8 +16,8 @@ impl Command for Source {
             .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "filename",
-                SyntaxShape::Filepath,
-                "The filepath to the script file to source.",
+                SyntaxShape::OneOf(vec![SyntaxShape::Filepath, SyntaxShape::Nothing]),
+                "The filepath to the script file to source (`null` for no-op).",
             )
             .category(Category::Core)
     }
@@ -42,6 +42,9 @@ impl Command for Source {
         call: &Call,
         input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
+        if call.get_parser_info(stack, "noop").is_some() {
+            return Ok(PipelineData::empty());
+        }
         // Note: two hidden positionals are used here that are injected by the parser:
         // 1. The block_id that corresponded to the 0th position
         // 2. The block_id_name that corresponded to the file name at the 0th position
@@ -107,6 +110,16 @@ impl Command for Source {
                 example: r#"source ./foo.nu; say-hi"#,
                 result: None,
             },
+            Example {
+                description: "Sourcing `null` is a no-op.",
+                example: r#"source null"#,
+                result: None,
+            },
+            Example {
+                description: "Source can be used with const variables.",
+                example: r#"const file = if $nu.is-interactive { "interactive.nu" } else { null }; source $file"#,
+                result: None,
+            }
         ]
     }
 }