Don't return NaN values for empty clusters

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "polars-tdigest"
-version = "0.1.3"
+version = "0.1.4"
 edition = "2021"
 
 [lib]

README.md

@@ -20,6 +20,20 @@ Install [Rust](https://rustup.rs/).
 
 In order to build the package, please run `maturin develop`. If you want to test performance, run `maturin develop --release`. 
 
+## Developing using cargo
+
+Cargo commands (e.g. `cargo build`, `cargo test`) don't work out of the box. 
+In order to use cargo instead of maturin for local development, remove `extension-module` from `cargo.toml`: 
+replace 
+```
+pyo3 = { version = "0.21.2", features = ["extension-module", "abi3-py38"] }
+```
+with 
+
+```
+pyo3 = { version = "0.21.2", features = ["abi3-py38"] }
+```
+
 ## Commit / Release
 
 Before committing and pushing your work, make sure to run

src/expressions.rs

@@ -176,9 +176,13 @@ fn tdigest(inputs: &[Series]) -> PolarsResult<Series> {
         _ => polars_bail!(InvalidOperation: "only supported for numerical types"),
     };
 
-    let t_global = TDigest::merge_digests(chunks);
+    let mut td_global = TDigest::merge_digests(chunks);
+    if td_global.is_empty() {
+        // Default value for TDigest contains NaNs that cause problems during serialization/deserailization
+        td_global = TDigest::new(Vec::new(), 100.0, 0.0, 0.0, 0.0, 0)
+    }
 
-    let td_json = serde_json::to_string(&t_global).unwrap();
+    let td_json = serde_json::to_string(&td_global).unwrap();
 
     let file = Cursor::new(&td_json);
     let df = JsonReader::new(file)