gitDirectoryURL can never be nil

It's asserted upon in libgit2.

Author: tiennou

ObjectiveGit/GTRepository.h

@@ -156,7 +156,7 @@ typedef NS_ENUM(NSInteger, GTRepositoryStateType) {
 /// Returns nil for a bare repository.
 @property (nonatomic, readonly, strong) NSURL * _Nullable fileURL;
 /// The file URL for the repository's .git directory.
-@property (nonatomic, readonly, strong) NSURL * _Nullable gitDirectoryURL;
+@property (nonatomic, readonly, strong) NSURL *gitDirectoryURL;
 
 /// Is this a bare repository (one without a working directory)?
 @property (nonatomic, readonly, getter = isBare) BOOL bare;

ObjectiveGit/GTRepository.m

@@ -641,10 +641,7 @@ - (NSURL *)fileURL {
 }
 
 - (NSURL *)gitDirectoryURL {
-	const char *path = git_repository_path(self.git_repository);
-	if (path == NULL) return nil;
-
-	return [NSURL fileURLWithPath:@(path) isDirectory:YES];
+	return [NSURL fileURLWithPath:@(git_repository_path(self.git_repository)) isDirectory:YES];
 }
 
 - (BOOL)isBare {