Handle empty paths when locating extensions (#7145)

Signed-off-by: Michael Kriese <michael.kriese@gmx.de>

Author: viceice

src/NuGet.Clients/NuGet.CommandLine/ExtensionLocator.cs

@@ -112,7 +112,7 @@ private static IEnumerable<string> FindAll(
             // and among other things breaks our build.
             // Consequently, we'll use a convention - only binaries ending in the name Extensions would be loaded.
             var nugetDirectory = Path.GetDirectoryName(typeof(Program).Assembly.Location);
-            if (nugetDirectory == null)
+            if (string.IsNullOrEmpty(nugetDirectory))
             {
                 return paths;
             }