added test cases for blank maps

tests/blank_map.rs

@@ -0,0 +1,43 @@
+use rosu_pp::{Beatmap, CatchPP, GameMode, ManiaPP, OsuPP, TaikoPP};
+
+#[test]
+fn osu() {
+    let map = Beatmap::default();
+    let _ = OsuPP::new(&map).calculate();
+}
+
+#[test]
+fn taiko() {
+    let mut map = Beatmap::default();
+
+    // convert
+    let _ = TaikoPP::new(&map).calculate();
+
+    // regular
+    map.mode = GameMode::Taiko;
+    let _ = TaikoPP::new(&map).calculate();
+}
+
+#[test]
+fn catch() {
+    let mut map = Beatmap::default();
+
+    // convert
+    let _ = CatchPP::new(&map).calculate();
+
+    // regular
+    map.mode = GameMode::Catch;
+    let _ = CatchPP::new(&map).calculate();
+}
+
+#[test]
+fn mania() {
+    let mut map = Beatmap::default();
+
+    // convert
+    let _ = ManiaPP::new(&map).calculate();
+
+    // regular
+    map.mode = GameMode::Mania;
+    let _ = ManiaPP::new(&map).calculate();
+}