Check nullCount in Series.toTypedArray (#207)

* check for empty validity mask in Series.toTypedArray

* add test for Series.toTypedArray null handling

* lint fix

__tests__/series.test.ts

@@ -656,6 +656,14 @@ describe("series", () => {
     const actual = s.round({ decimals: 2 });
     expect(actual).toSeriesEqual(expected);
   });
+  test("toTypedArray handles nulls", () => {
+    const s = pl.Series("ints and nulls", [1, 2, 3, null, 5], pl.UInt8);
+    expect(() => s.toTypedArray()).toThrow();
+    expect(() => s.dropNulls().toTypedArray()).not.toThrow();
+    expect(s.dropNulls().toTypedArray()).toStrictEqual(
+      new Uint8Array([1, 2, 3, 5]),
+    );
+  });
 });
 describe("comparators & math", () => {
   test("add/plus", () => {

polars/series/index.ts

@@ -1710,7 +1710,7 @@ export function _Series(_s: any): Series {
       return _s.toArray();
     },
     toTypedArray() {
-      if (!this.hasValidity()) {
+      if (!this.hasValidity() || this.nullCount() === 0) {
         return _s.toTypedArray();
       }
       throw new Error("data contains nulls, unable to convert to TypedArray");