[gardening] Remove redundant nil-initialization of optional variable

From the Swift documentation:

"If you define an optional variable without providing a default value,
 the variable is automatically set to nil for you."

test/stdlib/PrintStruct.swift

@@ -33,7 +33,7 @@ PrintTests.test("Printable") {
 
 PrintTests.test("custom string convertible structs") {
   struct Wrapper : CustomStringConvertible {
-    var x: CustomStringConvertible? = nil
+    var x: CustomStringConvertible?
     
     var description: String {
       return "Wrapper(\(x.debugDescription))"