test: add initial coverage for builtin/array_wbtest.mbt

remove unneeded check

builtin/array.mbt

@@ -273,9 +273,6 @@ pub fn clear[T](self : Array[T]) -> Unit {
 /// assert_eq!(v2, [4, 5, 6])
 /// ```
 pub fn map[T, U](self : Array[T], f : (T) -> U) -> Array[U] {
-  if self.length() == 0 {
-    return []
-  }
   let arr = Array::make_uninit(self.length())
   for i, v in self {
     arr.unsafe_set(i, f(v))

builtin/array_wbtest.mbt

@@ -0,0 +1,18 @@
+// Copyright 2024 International Digital Economy Academy
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+test {
+  let empty : Array[Unit] = Array::make_uninit(0)
+  inspect!(empty, content="[]")
+}