Return empty array if length is zero (#16529)

* Return empty array if length is zero

* Return true

* Add support for portable system

* Use portable span helper instead of Array.Empty

* Move to end

* Remove else

* Return empty array if length is 0 in MemoryMarshal

Author: kasper3

src/mscorlib/shared/System/Memory.cs

@@ -329,6 +329,16 @@ public bool TryGetArray(out ArraySegment<T> arraySegment)
                 return true;
             }
 
+            if (_length == 0)
+            {
+#if FEATURE_PORTABLE_SPAN
+                arraySegment = new ArraySegment<T>(SpanHelpers.PerTypeValues<T>.EmptyArray);
+#else
+                arraySegment = ArraySegment<T>.Empty;
+#endif // FEATURE_PORTABLE_SPAN
+                return true;
+            }
+
             arraySegment = default(ArraySegment<T>);
             return false;
         }

src/mscorlib/shared/System/Runtime/InteropServices/MemoryMarshal.cs

@@ -35,6 +35,16 @@ public static bool TryGetArray<T>(ReadOnlyMemory<T> readOnlyMemory, out ArraySeg
                 return true;
             }
 
+            if (length == 0)
+            {
+#if FEATURE_PORTABLE_SPAN
+                arraySegment = new ArraySegment<T>(SpanHelpers.PerTypeValues<T>.EmptyArray);
+#else
+                arraySegment = ArraySegment<T>.Empty;
+#endif // FEATURE_PORTABLE_SPAN
+                return true;
+            }
+
             arraySegment = default;
             return false;
         }