return null for first, last, nth on error

Instead of empty string, return null value that's considered undefined
(and can be piped to the default filter) if the array is empty for the
filters first, last and nth. For nth, also change the out-of-bounds
value from empty string to null.

Closes #534, and is a breaking change.

docs/content/docs/_index.md

@@ -819,15 +819,15 @@ before `striptags`.
 
 #### first
 Returns the first element of an array.
-If the array is empty, returns empty string.
+If the array is empty, returns null (undefined) that can be chained to the default filter.
 
 #### last
 Returns the last element of an array.
-If the array is empty, returns empty string.
+If the array is empty, returns null (undefined) that can be chained to the default filter.
 
 #### nth
-Returns the nth element of an array.§
-If the array is empty, returns empty string.
+Returns the nth element of an array.
+If the array is empty or if the nth is out of bounds, returns null (undefined) that can be chained to the default filter.
 It takes a required `n` argument, corresponding to the 0-based index you want to get.
 
 Example: `{{ value | nth(n=2) }}`

src/builtins/filters/array.rs

@@ -7,45 +7,45 @@ use crate::filter_utils::{get_sort_strategy_for_type, get_unique_strategy_for_ty
 use serde_json::value::{to_value, Map, Value};
 
 /// Returns the nth value of an array
-/// If the array is empty, returns empty string
+/// If the array is empty, returns null
 pub fn nth(value: &Value, args: &HashMap<String, Value>) -> Result<Value> {
     let arr = try_get_value!("nth", "value", Vec<Value>, value);
 
     if arr.is_empty() {
-        return Ok(to_value("").unwrap());
+        return Ok(Value::Null);
     }
 
     let index = match args.get("n") {
         Some(val) => try_get_value!("nth", "n", usize, val),
         None => return Err(Error::msg("The `nth` filter has to have an `n` argument")),
     };
 
-    Ok(arr.get(index).unwrap_or(&to_value("").unwrap()).to_owned())
+    Ok(arr.get(index).map(|x| x.to_owned()).unwrap_or(Value::Null))
 }
 
 /// Returns the first value of an array
-/// If the array is empty, returns empty string
+/// If the array is empty, returns null
 pub fn first(value: &Value, _: &HashMap<String, Value>) -> Result<Value> {
     let mut arr = try_get_value!("first", "value", Vec<Value>, value);
 
     if arr.is_empty() {
-        Ok(to_value("").unwrap())
+        Ok(Value::Null)
     } else {
         Ok(arr.swap_remove(0))
     }
 }
 
 /// Returns the last value of an array
-/// If the array is empty, returns empty string
+/// If the array is empty, returns null
 pub fn last(value: &Value, _: &HashMap<String, Value>) -> Result<Value> {
     let mut arr = try_get_value!("last", "value", Vec<Value>, value);
 
-    Ok(arr.pop().unwrap_or_else(|| to_value("").unwrap()))
+    Ok(arr.pop().unwrap_or(Value::Null))
 }
 
 /// Joins all values in the array by the `sep` argument given
 /// If no separator is given, it will use `""` (empty string) as separator
-/// If the array is empty, returns empty string
+/// If the array is empty, returns null
 pub fn join(value: &Value, args: &HashMap<String, Value>) -> Result<Value> {
     let arr = try_get_value!("join", "value", Vec<Value>, value);
     let sep = match args.get("sep") {
@@ -319,7 +319,7 @@ mod tests {
         args.insert("n".to_string(), to_value(1).unwrap());
         let result = nth(&to_value(&v).unwrap(), &args);
         assert!(result.is_ok());
-        assert_eq!(result.unwrap(), to_value("").unwrap());
+        assert_eq!(result.unwrap(), Value::Null);
     }
 
     #[test]
@@ -335,7 +335,7 @@ mod tests {
 
         let result = first(&to_value(&v).unwrap(), &HashMap::new());
         assert!(result.is_ok());
-        assert_eq!(result.ok().unwrap(), to_value("").unwrap());
+        assert_eq!(result.ok().unwrap(), Value::Null);
     }
 
     #[test]
@@ -351,7 +351,7 @@ mod tests {
 
         let result = last(&to_value(&v).unwrap(), &HashMap::new());
         assert!(result.is_ok());
-        assert_eq!(result.ok().unwrap(), to_value("").unwrap());
+        assert_eq!(result.ok().unwrap(), Value::Null);
     }
 
     #[test]