Make Repository.IsValid() return false on empty paths

Fix #818

Author: ptr1120

LibGit2Sharp.Tests/RepositoryFixture.cs

@@ -57,6 +57,26 @@ public void CanCheckIfADirectoryLeadsToAValidRepository()
             Assert.False(Repository.IsValid(scd.DirectoryPath));
         }
 
+
+        [Fact]
+        public void IsValidWithNullPathThrows()
+        {
+            Assert.Throws<ArgumentNullException>(() => Repository.IsValid(null));
+        }
+
+        [Fact]
+        public void IsNotValidWithEmptyPath()
+        {
+            Assert.False(Repository.IsValid(string.Empty));
+        }
+
+        [Fact]
+        public void IsValidWithValidPath()
+        {
+            string repoPath = InitNewRepository();
+            Assert.True(Repository.IsValid(repoPath));
+        }
+
         [Fact]
         public void CanCreateStandardRepo()
         {

LibGit2Sharp/Repository.cs

@@ -155,7 +155,12 @@ public Repository(string path, RepositoryOptions options)
         /// <returns>True if a repository can be resolved through this path; false otherwise</returns>
         static public bool IsValid(string path)
         {
-            Ensure.ArgumentNotNullOrEmptyString(path, "path");
+            Ensure.ArgumentNotNull(path, "path");
+
+            if (string.IsNullOrWhiteSpace(path))
+            {
+                return false;
+            }
 
             try
             {