Removed 'nullable' attribute of BranchAdvancedLogCallback param

[NidhiDixit09]

Reference

SDK-2537 -- Fix enableLogging API error - Ambiguous use of 'enableLogging'
https://branch.atlassian.net/browse/SDK-2537

Summary

Following are two overloaded versions of static API enableLoggingAtLevel and both had the second param nullable attribute. This resulted in two methods with same name and same param.

+ (void)enableLoggingAtLevel:(BranchLogLevel)logLevel withCallback:(nullable BranchLogCallback)callback;
+ (void)enableLoggingAtLevel:(BranchLogLevel)logLevel withAdvancedCallback:(nullable  BranchAdvancedLogCallback)callback;

Type Of Change

• Bug fix (non-breaking change which fixes an issue)

Testing Instructions

In a test app, call API Branch.enableLogging(at: .verbose). Compile app to see we no longer get Build error.

cc @BranchMetrics/saas-sdk-devs for visibility.