Respect nullable returns in various methods

Some properties could return nil where they are not annotated as such.

Author: slavikus

ObjectiveGit/GTCredential.m

@@ -109,7 +109,7 @@ int GTCredentialAcquireCallback(git_cred **git_cred, const char *url, const char
 		return GIT_ERROR;
 	}
 
-	NSString *URL = (url != NULL ? @(url) : nil);
+	NSString *URL = (url != NULL ? @(url) : @"");
 	NSString *userName = (username_from_url != NULL ? @(username_from_url) : nil);
 
 	GTCredential *cred = [provider credentialForType:(GTCredentialType)allowed_types URL:URL userName:userName];

ObjectiveGit/GTIndex.h

@@ -69,7 +69,7 @@ NS_ASSUME_NONNULL_BEGIN
 /// error      - If not NULL, set to any error that occurs.
 ///
 /// Returns the loaded index, or nil if an error occurred.
-+ (instancetype)indexWithFileURL:(NSURL *)fileURL repository:(GTRepository *)repository error:(NSError **)error;
++ (nullable instancetype)indexWithFileURL:(NSURL *)fileURL repository:(GTRepository *)repository error:(NSError **)error;
 
 - (instancetype)init NS_UNAVAILABLE;
 

ObjectiveGit/GTIndexEntry.h

@@ -61,7 +61,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (const git_index_entry *)git_index_entry __attribute__((objc_returns_inner_pointer));
 
 /// The entry's index. This may be nil if nil is passed in to -initWithGitIndexEntry:
-@property (nonatomic, strong, readonly) GTIndex *index;
+@property (nonatomic, strong, readonly, nullable) GTIndex *index;
 
 /// The repository-relative path for the entry.
 @property (nonatomic, readonly, copy) NSString *path;

ObjectiveGit/GTOID.h

@@ -16,7 +16,7 @@ NS_ASSUME_NONNULL_BEGIN
 @interface GTOID : NSObject <NSCopying>
 
 /// The SHA pointed to by the OID.
-@property (nonatomic, readonly, copy) NSString *SHA;
+@property (nonatomic, readonly, copy, nullable) NSString *SHA;
 
 /// Is the OID all zero? This usually indicates that the object has not been
 /// inserted into the ODB yet.

ObjectiveGit/GTReference.h

@@ -52,7 +52,7 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, readonly, strong) GTRepository *repository;
 @property (nonatomic, readonly) GTReferenceType referenceType;
 @property (nonatomic, readonly) const git_oid *git_oid;
-@property (nonatomic, strong, readonly) GTOID *OID;
+@property (nonatomic, strong, readonly, nullable) GTOID *OID;
 
 /// Whether this is a remote-tracking branch.
 @property (nonatomic, readonly, getter = isRemote) BOOL remote;
@@ -78,10 +78,10 @@ NS_ASSUME_NONNULL_BEGIN
 - (git_reference *)git_reference __attribute__((objc_returns_inner_pointer));
 
 /// The target (either GTObject or GTReference) to which the reference points.
-@property (nonatomic, readonly, copy) id unresolvedTarget;
+@property (nonatomic, readonly, copy, nullable) id unresolvedTarget;
 
 /// The resolved object to which the reference points.
-@property (nonatomic, readonly, copy) id resolvedTarget;
+@property (nonatomic, readonly, copy, nullable) id resolvedTarget;
 
 /// The last direct reference in a chain
 @property (nonatomic, readonly, copy) GTReference *resolvedReference;

ObjectiveGit/GTRepository.h

@@ -177,7 +177,7 @@ typedef NS_ENUM(NSInteger, GTRepositoryStateType) {
 @interface GTRepository : NSObject
 
 /// The file URL for the repository's working directory.
-@property (nonatomic, readonly, strong) NSURL *fileURL;
+@property (nonatomic, readonly, strong, nullable) NSURL *fileURL;
 /// The file URL for the repository's .git directory.
 @property (nonatomic, readonly, strong, nullable) NSURL *gitDirectoryURL;
 

ObjectiveGit/GTSignature.h

@@ -42,10 +42,10 @@ NS_ASSUME_NONNULL_BEGIN
 @property (nonatomic, readonly, copy, nullable) NSString *email;
 
 /// The time when the action happened.
-@property (nonatomic, readonly, strong) NSDate *time;
+@property (nonatomic, readonly, strong, nullable) NSDate *time;
 
 /// The time zone that `time` should be interpreted relative to.
-@property (nonatomic, readonly, copy) NSTimeZone *timeZone;
+@property (nonatomic, readonly, copy, nullable) NSTimeZone *timeZone;
 
 /// Initializes the receiver with the given signature.
 ///