fix: support zero-byte uploads in FilePut

Reading the body into a zero-length buffer returned io.EOF on an
already-exhausted reader, so empty files and directory markers always
failed. Use io.ReadFull and send an explicit Content-Length: 0 for
empty bodies instead of falling back to chunked encoding. Also fix an
error variable mix-up in the NewRequest error check.

object_zerobody_test.go

@@ -0,0 +1,46 @@
+package simples3
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// A zero-byte upload (empty file, directory marker) must succeed: FilePut
+// used to fail with io.EOF because it read the body into a zero-length
+// buffer before sending the request.
+func TestS3_FilePut_EmptyBody(t *testing.T) {
+	var gotBody []byte
+	var gotContentLength string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotContentLength = r.Header.Get("Content-Length")
+		gotBody, _ = io.ReadAll(r.Body)
+		w.Header().Set("ETag", `"d41d8cd98f00b204e9800998ecf8427e"`)
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer srv.Close()
+
+	s3 := New("us-east-1", "test-access", "test-secret")
+	s3.SetEndpoint(srv.URL)
+
+	resp, err := s3.FilePut(UploadInput{
+		Bucket:      "test-bucket",
+		ObjectKey:   "empty-dir/",
+		ContentType: "application/x-directory",
+		Body:        bytes.NewReader(nil),
+	})
+	if err != nil {
+		t.Fatalf("S3.FilePut() with empty body returned error: %v", err)
+	}
+	if resp.ETag == "" {
+		t.Errorf("S3.FilePut() returned empty response: %+v", resp)
+	}
+	if gotContentLength != "0" {
+		t.Errorf("Content-Length = %q, want %q", gotContentLength, "0")
+	}
+	if len(gotBody) != 0 {
+		t.Errorf("request body = %q, want empty", gotBody)
+	}
+}