fix(pii): Early return if no text left (#1957)

If a list of chunks consists only of replacements, the following debug
assertion at the end of `apply_replacement_chunks` might fail (see
test):

https://github.com/getsentry/relay/blob/a8ee53cc26b8d7bd10cfa919e672894edbd65916/relay-general/src/pii/processor.rs#L346

Add an early return to prevent this failure.

This popped up in a test case in #1951.

Author: jjbayer

relay-general/src/pii/processor.rs

@@ -263,13 +263,22 @@ fn apply_regex_to_chunks<'a>(
     // on the chunks, but the `regex` crate does not support that.
 
     let mut search_string = String::new();
+    let mut has_text = false;
     for chunk in &chunks {
         match chunk {
-            Chunk::Text { text } => search_string.push_str(&text.replace('\x00', "")),
+            Chunk::Text { text } => {
+                has_text = true;
+                search_string.push_str(&text.replace('\x00', ""));
+            }
             Chunk::Redaction { .. } => search_string.push('\x00'),
         }
     }
 
+    if !has_text {
+        // Nothing to replace.
+        return chunks;
+    }
+
     // Early exit if this regex does not match and return the original chunks.
     let mut captures_iter = regex.captures_iter(&search_string).peekable();
     if captures_iter.peek().is_none() {
@@ -1032,6 +1041,30 @@ mod tests {
         assert_eq!(chunks, res);
     }
 
+    #[test]
+    fn test_replace_replaced_text_anything() {
+        let chunks = vec![Chunk::Redaction {
+            text: "[Filtered]".into(),
+            rule_id: "@password:filter".into(),
+            ty: RemarkType::Substituted,
+        }];
+        let rule = RuleRef {
+            id: "@anything:filter".into(),
+            origin: "@anything:filter".into(),
+            ty: RuleType::Anything,
+            redaction: Redaction::Replace(ReplaceRedaction {
+                text: "[Filtered]".into(),
+            }),
+        };
+        let res = apply_regex_to_chunks(
+            chunks.clone(),
+            &rule,
+            &Regex::new(r#".*"#).unwrap(),
+            ReplaceBehavior::Groups(smallvec::smallvec![0]),
+        );
+        assert_eq!(chunks, res);
+    }
+
     #[test]
     fn test_scrub_span_data_not_scrubbed() {
         let mut span = Annotated::new(Span {