[bitboard] Implement From<Option<Square>> for BitBoard

Return an empty BitBoard if the input is None.

bitboard/src/bitboard.rs

@@ -164,6 +164,12 @@ impl From<File> for BitBoard {
     }
 }
 
+impl From<Option<Square>> for BitBoard {
+    fn from(value: Option<Square>) -> Self {
+        value.map_or(BitBoard::EMPTY, Into::<BitBoard>::into)
+    }
+}
+
 impl From<Rank> for BitBoard {
     fn from(value: Rank) -> Self {
         library::FILES[*value.as_index() as usize]