implement guarded array first

Author: TheIronBorn

Select Minimum Maximum/Minimum.swift

@@ -3,11 +3,8 @@
  */
 
 func minimum<T: Comparable>(_ array: [T]) -> T? {
-    guard !array.isEmpty else {
-        return nil
-    }
-
-    var minimum = array.first!
+    guard minimum = array.first else { return nil }
+ 
     for element in array.dropFirst() {
         minimum = element < minimum ? element : minimum
     }