properly insert into empty strings

src/builtin/string.rs

@@ -176,6 +176,10 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
                 QuoteKind::None => QuoteKind::None,
             };
 
+            if s1.is_empty() {
+                return Ok(Value::Ident(substr, quotes));
+            }
+
             let len = s1.len();
 
             // Insert substring at char position, rather than byte position

tests/strings.rs

@@ -141,6 +141,11 @@ test!(
 );
 test!(
     str_insert_empty_string,
+    "a {\n  color: str-insert(\"\", \"abcd\", 4);\n}\n",
+    "a {\n  color: \"abcd\";\n}\n"
+);
+test!(
+    str_insert_empty_substring,
     "a {\n  color: str-insert(abcd, \"\", 4);\n}\n",
     "a {\n  color: abcd;\n}\n"
 );