fix: incorrect thrown exception when calling Path.GetFullPath with white-space (#1030)

Throw the correct `ArgumentException` instead of a `NullReferenceException` when calling Path.GetFullPath with a WhiteSpace string.

Author: vbreuss

src/TestableIO.System.IO.Abstractions.TestingHelpers/MockPath.cs

@@ -39,7 +39,7 @@ public override string GetFullPath(string path)
                 throw new ArgumentNullException(nameof(path), StringResources.Manager.GetString("VALUE_CANNOT_BE_NULL"));
             }
 
-            if (path.Length == 0)
+            if (string.IsNullOrWhiteSpace(path))
             {
                 throw CommonExceptions.PathIsNotOfALegalForm(nameof(path));
             }

tests/TestableIO.System.IO.Abstractions.TestingHelpers.Tests/MockPathTests.cs

@@ -266,6 +266,16 @@ public void GetFullPath_EmptyValue_ShouldThrowArgumentException()
             Assert.Throws<ArgumentException>(action);
         }
 
+        [Test]
+        public void GetFullPath_WithWhiteSpace_ShouldThrowArgumentException()
+        {
+            var mockFileSystem = new MockFileSystem();
+
+            TestDelegate action = () => mockFileSystem.Path.GetFullPath("  ");
+
+            Assert.Throws<ArgumentException>(action);
+        }
+
         [Test]
         public void GetFullPath_WithMultipleDirectorySeparators_ShouldReturnTheNormalizedForm()
         {