Add simple test for empty span check.

Author: wo80

CSparse.Tests/Complex/Factorization/SparseLUTest.cs

@@ -3,6 +3,7 @@ namespace CSparse.Tests.Complex.Factorization
     using CSparse.Complex;
     using CSparse.Complex.Factorization;
     using NUnit.Framework;
+    using System;
     using Complex = System.Numerics.Complex;
 
     public class SparseLUTest
@@ -30,6 +31,14 @@ public void TestSolve()
             A.Multiply(-1.0, x, 1.0, r);
 
             Assert.That(Vector.Norm(r.Length, r) < EPS, Is.True);
+
+            // Test exceptions:
+
+            var e1 = Assert.Throws<ArgumentNullException>(() => lu.Solve(b, null));
+            var e2 = Assert.Throws<ArgumentNullException>(() => lu.Solve(null, x));
+
+            Assert.That(e1.ParamName, Is.EqualTo("result"));
+            Assert.That(e2.ParamName, Is.EqualTo("input"));
         }
 
         [Test]

CSparse.Tests/Double/Factorization/SparseLUTest.cs

@@ -3,6 +3,7 @@ namespace CSparse.Tests.Double.Factorization
     using CSparse.Double;
     using CSparse.Double.Factorization;
     using NUnit.Framework;
+    using System;
 
     public class SparseLUTest
     {
@@ -29,6 +30,14 @@ public void TestSolve()
             A.Multiply(-1.0, x, 1.0, r);
 
             Assert.That(Vector.Norm(r.Length, r) < EPS, Is.True);
+
+            // Test exceptions:
+
+            var e1 = Assert.Throws<ArgumentNullException>(() => lu.Solve(b, null));
+            var e2 = Assert.Throws<ArgumentNullException>(() => lu.Solve(null, x));
+
+            Assert.That(e1.ParamName, Is.EqualTo("result"));
+            Assert.That(e2.ParamName, Is.EqualTo("input"));
         }
 
         [Test]

CSparse/Complex/Factorization/SparseLU.cs

@@ -153,9 +153,9 @@ public void Solve(ReadOnlySpan<Complex> input, Span<Complex> result)
         /// <param name="result">The left hand side vector, <c>x</c>.</param>
         public void SolveTranspose(ReadOnlySpan<Complex> input, Span<Complex> result)
         {
-            if (input == null) throw new ArgumentNullException(nameof(input));
+            if (input.IsEmpty) throw new ArgumentNullException(nameof(input));
 
-            if (result == null) throw new ArgumentNullException(nameof(result));
+            if (result.IsEmpty) throw new ArgumentNullException(nameof(result));
 
             var x = this.temp;