Add nullability to Logger+ASL.

Foundation/GTMLogger+ASL.h

@@ -20,6 +20,8 @@
 #import <asl.h>
 #import "GTMLogger.h"
 
+NS_ASSUME_NONNULL_BEGIN
+
 // GTMLogger (GTMLoggerASLAdditions)
 //
 // Adds a convenience creation method that allows you to get a standard
@@ -62,15 +64,15 @@
 // Returns an autoreleased GTMLogASLWriter instance that uses an instance of
 // GTMLoggerASLClient and the supplied facility. See asl_open(3) for a
 // discusssion of ASL facility strings.
-+ (instancetype)aslWriterWithFacility:(NSString *)facility;
++ (instancetype)aslWriterWithFacility:(nullable NSString *)facility;
 
 // Designated initializer. Uses instances of the specified |clientClass| to talk
 // to the ASL system. All logs from this method will use |facility| as the ASL
 // log facility. This method is typically only useful for testing. Users
 // should generally NOT use this method to get an instance. Instead, simply use
 // the +aslWriter or +aslWriterWithFacility: methods to obtain an instance.
 - (instancetype)initWithClientClass:(Class)clientClass
-                           facility:(NSString *)facility;
+                           facility:(nullable NSString *)facility;
 
 @end  // GTMLogASLWriter
 
@@ -96,10 +98,13 @@
   aslmsg msgOptions_;
 }
 
-// Designated initializer, |facility| is supplied to asl_open().
-- (instancetype)initWithFacility:(NSString *)facility;
+// Designated initializer, |facility| is supplied to asl_open(). Can fail
+// if asl_open fails.
+- (nullable instancetype)initWithFacility:(nullable NSString *)facility;
 
 // Sends the given string to ASL at the specified ASL log |level|.
 - (void)log:(NSString *)msg level:(int)level;
 
 @end
+
+NS_ASSUME_NONNULL_END

Foundation/GTMLogger+ASL.m

@@ -34,15 +34,18 @@ + (instancetype)standardLoggerWithASL {
 @implementation GTMLogASLWriter
 
 + (instancetype)aslWriter {
-  return [[[self alloc] initWithClientClass:nil facility:nil] autorelease];
+  return [[[self alloc] initWithClientClass:[GTMLoggerASLClient class]
+                                   facility:nil] autorelease];
 }
 
 + (instancetype)aslWriterWithFacility:(NSString *)facility {
-  return [[[self alloc] initWithClientClass:nil facility:facility] autorelease];
+  return [[[self alloc] initWithClientClass:[GTMLoggerASLClient class]
+                                   facility:facility] autorelease];
 }
 
 - (instancetype)init {
-  return [self initWithClientClass:nil facility:nil];
+  return [self initWithClientClass:[GTMLoggerASLClient class]
+                          facility:nil];
 }
 
 - (instancetype)initWithClientClass:(Class)clientClass