check for datafile after running block

Author: wangjoshuah

OptimizelySDKCore/OptimizelySDKCore/OPTLYProjectConfigBuilder.m

@@ -27,12 +27,12 @@ - (id) init {
 }
 
 - (id)initWithBlock:(OPTLYProjectConfigBuilderBlock)block {
-    if (!_datafile) {
-        return nil;
-    }
     self = [super init];
     if (self != nil) {
         block(self);
+        if (!_datafile) {
+            return nil;
+        }
     }
     return self;
 }