ENH: Add static_assert(VLength > 0) to FixedArray

Zero-length arrays were never actively supported by `itk::FixedArray`.
`FixedArray` uses a C array, so if it would compile at all, it would depend on a
non-standard compiler extension.

It appears unnecessary to support zero-length, given one of the main purposes of
`itk::FixedArray`: being a common base class of Point, Vector, CovariantVector,
RGBAPixel, RGBAPixel and SymmetricSecondRankTensor.

Author: N-Dekker

Modules/Core/Common/include/itkFixedArray.h

@@ -52,6 +52,8 @@ namespace itk
 template <typename TValue, unsigned int VLength = 3>
 class ITK_TEMPLATE_EXPORT FixedArray
 {
+  static_assert(VLength > 0);
+
 public:
   /** Length constant */
   static constexpr unsigned int Length = VLength;