Fix inc plugin

src/parser/registry.rs

@@ -32,6 +32,14 @@ impl PositionalType {
         PositionalType::Mandatory(name.to_string(), SyntaxType::Any)
     }
 
+    pub fn optional(name: &str, ty: SyntaxType) -> PositionalType {
+        PositionalType::Optional(name.to_string(), ty)
+    }
+
+    pub fn optional_any(name: &str) -> PositionalType {
+        PositionalType::Optional(name.to_string(), SyntaxType::Any)
+    }
+
     pub fn mandatory_block(name: &str) -> PositionalType {
         PositionalType::Mandatory(name.to_string(), SyntaxType::Block)
     }

src/plugins/inc.rs

@@ -17,7 +17,7 @@ impl Plugin for Inc {
     fn config(&mut self) -> Result<CommandConfig, ShellError> {
         Ok(CommandConfig {
             name: "inc".to_string(),
-            positional: vec![PositionalType::mandatory_any("Increment")],
+            positional: vec![PositionalType::optional_any("Increment")],
             is_filter: true,
             is_sink: false,
             named: IndexMap::new(),