Handle csproj "Remove" globs

MSBuild Item returns empty strings if an attribute isn't set (which
caused an IndexOutOfRangeException in NormalizePath).

We were treating Excludes incorrectly, Remove directives provide the
intended behaviour in the auto-including csproj format.

modules/mono/editor/GodotTools/GodotTools.Core/StringExtensions.cs

@@ -23,6 +23,9 @@ namespace GodotTools.Core
 
         public static string NormalizePath(this string path)
         {
+            if (string.IsNullOrEmpty(path))
+                return path;
+
             bool rooted = path.IsAbsolutePath();
 
             path = path.Replace('\\', '/');

modules/mono/editor/GodotTools/GodotTools.ProjectEditor/ProjectUtils.cs

@@ -61,10 +61,9 @@ namespace GodotTools.ProjectEditor
                 if (item.ItemType != itemType)
                     continue;
 
-                string normalizedExclude = item.Exclude.NormalizePath();
-
-                var glob = MSBuildGlob.Parse(normalizedExclude);
+                string normalizedRemove = item.Remove.NormalizePath();
 
+                var glob = MSBuildGlob.Parse(normalizedRemove);
                 excluded.AddRange(includedFiles.Where(includedFile => glob.IsMatch(includedFile)));
             }