Providers: don't fail if hint path is null

src/Providers.CUDA/LinearAlgebra/CudaLinearAlgebraProvider.cs

@@ -47,7 +47,7 @@ namespace MathNet.Numerics.Providers.CUDA.LinearAlgebra
         /// <param name="hintPath">Hint path where to look for the native binaries</param>
         internal CudaLinearAlgebraProvider(string hintPath)
         {
-            _hintPath = Path.GetFullPath(hintPath);
+            _hintPath = hintPath != null ? Path.GetFullPath(hintPath) : null;
         }
 
         /// <summary>

src/Providers.MKL/FourierTransform/MklFourierTransformProvider.cs

@@ -54,7 +54,7 @@ namespace MathNet.Numerics.Providers.MKL.FourierTransform
         /// <param name="hintPath">Hint path where to look for the native binaries</param>
         internal MklFourierTransformProvider(string hintPath)
         {
-            _hintPath = Path.GetFullPath(hintPath);
+            _hintPath = hintPath != null ? Path.GetFullPath(hintPath) : null;
         }
 
         /// <summary>

src/Providers.MKL/LinearAlgebra/MklLinearAlgebraProvider.cs

@@ -71,7 +71,7 @@ namespace MathNet.Numerics.Providers.MKL.LinearAlgebra
         /// <param name="accuracy">VML accuracy mode.</param>
         internal MklLinearAlgebraProvider(string hintPath, MklConsistency consistency, MklPrecision precision, MklAccuracy accuracy)
         {
-            _hintPath = Path.GetFullPath(hintPath);
+            _hintPath = hintPath != null ? Path.GetFullPath(hintPath) : null;
             _consistency = consistency;
             _precision = precision;
             _accuracy = accuracy;

src/Providers.MKL/SparseSolver/MklSparseSolverProvider.cs

@@ -19,7 +19,7 @@ namespace MathNet.Numerics.Providers.MKL.SparseSolver
         /// <param name="hintPath">Hint path where to look for the native binaries</param>
         internal MklSparseSolverProvider(string hintPath)
         {
-            _hintPath = Path.GetFullPath(hintPath);
+            _hintPath = hintPath != null ? Path.GetFullPath(hintPath) : null;
         }
 
         /// <summary>

src/Providers.OpenBLAS/LinearAlgebra/OpenBlasLinearAlgebraProvider.cs

@@ -63,7 +63,7 @@ namespace MathNet.Numerics.Providers.OpenBLAS.LinearAlgebra
         /// <param name="hintPath">Hint path where to look for the native binaries</param>
         internal OpenBlasLinearAlgebraProvider(string hintPath)
         {
-            _hintPath = Path.GetFullPath(hintPath);
+            _hintPath = hintPath != null ? Path.GetFullPath(hintPath) : null;
         }
 
         /// <summary>