csv: fix parse error of last empty field on unquoted line (#10083)

vlib/encoding/csv/reader.v

@@ -134,7 +134,7 @@ fn (mut r Reader) read_record() ?[]string {
 			need_read = false
 			keep_raw = false
 		}
-		if line[0] != `"` { // not quoted
+		if line.len == 0 || line[0] != `"` { // not quoted
 			j := line.index(r.delimiter.ascii_str()) or {
 				// last
 				fields << line[..line.len]
@@ -160,9 +160,10 @@ fn (mut r Reader) read_record() ?[]string {
 			if next == r.delimiter {
 				fields << line[..j]
 				if j + 2 == line.len {
-					break
+					line = ''
+				} else {
+					line = line[j + 2..]
 				}
-				line = line[j + 2..]
 				continue
 			}
 		}

vlib/encoding/csv/reader_test.v

@@ -111,11 +111,50 @@ fn test_last_field_empty() {
 		} else if row_count == 3 {
 			assert row[0] == 'two'
 			assert row[1] == 'second'
+			assert row[2] == ''
 		}
 	}
 	assert row_count == 3
 }
 
+fn test_empty_fields_no_quotes() {
+	data := '1,2,3,4\n,6,7,8\n9,,11,12\n13,14,,16\n17,18,19,\n'
+
+	mut csv_reader := csv.new_reader(data)
+	mut row_count := 0
+	for {
+		row := csv_reader.read() or { break }
+		row_count++
+		if row_count == 1 {
+			assert row[0] == '1'
+			assert row[1] == '2'
+			assert row[2] == '3'
+			assert row[3] == '4'
+		} else if row_count == 2 {
+			assert row[0] == ''
+			assert row[1] == '6'
+			assert row[2] == '7'
+			assert row[3] == '8'
+		} else if row_count == 3 {
+			assert row[0] == '9'
+			assert row[1] == ''
+			assert row[2] == '11'
+			assert row[3] == '12'
+		} else if row_count == 4 {
+			assert row[0] == '13'
+			assert row[1] == '14'
+			assert row[2] == ''
+			assert row[3] == '16'
+		} else if row_count == 5 {
+			assert row[0] == '17'
+			assert row[1] == '18'
+			assert row[2] == '19'
+			assert row[3] == ''
+		}
+	}
+	assert row_count == 5
+}
+
 fn test_empty_line() {
 	data := '"name","description","value"\n\n\n"one","first","1"\n\n"two","second",\n'
 	mut csv_reader := csv.new_reader(data)