make sure we don't crash with bad access error when we initialize core optimizely with nil builder

Author: wangjoshuah

OptimizelySDKCore/OptimizelySDKCore/OPTLYBuilder.m

@@ -33,6 +33,10 @@ - (id)init {
 }
 
 - (id)initWithBlock:(OPTLYBuilderBlock)block {
+    // check for nil block
+    if (block == nil) {
+        return nil;
+    }
     self = [super init];
     if (self != nil) {
         block(self);

OptimizelySDKCore/OptimizelySDKCoreTests/OPTLYBuilderTest.m

@@ -108,4 +108,9 @@ - (void)testBuilderCanAssignLogger {
     XCTAssertEqual(logger, customOptimizely.logger, @"Should be the same object with custom builder");
 }
 
+- (void)testInitializationWithoutBuilder {
+    Optimizely *optimizely = [Optimizely initWithBuilderBlock:nil];
+    XCTAssertNil(optimizely);
+}
+
 @end