gitDirectoryURL can never be nil

It's asserted upon in libgit2.

Author: tiennou

ObjectiveGit/GTRepository.h

@@ -159,7 +159,7 @@ typedef NS_ENUM(NSInteger, GTRepositoryStateType) {
 @property (nonatomic, readonly, strong) NSURL * _Nullable fileURL DEPRECATED_MSG_ATTRIBUTE("use -workingDirectoryURL");
 
 /// 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

@@ -645,10 +645,7 @@ - (NSURL *)workingDirectoryURL {
 }
 
 - (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 {