Make SentryException value and type nullable

Description

Only one of SentryException.value or SentryException.type is required to be nonnull; see Relay. The value can already be nil when converting an NSException to a SentryException in the client as NSException.reason is nullable.

sentry-cocoa/Sources/Sentry/SentryClient.m

Lines 208 to 210 in cf97209

    
           SentryEvent *event = [[SentryEvent alloc] initWithLevel:kSentryLevelError]; 
        
           SentryException *sentryException = [[SentryException alloc] initWithValue:exception.reason 
        
                                                                                type:exception.name];

Therefore, we should make both value and type nullable readonly and add constructors to ensure either one of the two is set. Consider converting this class to Swift when doing these changes.

	SentryEvent *event = [[SentryEvent alloc] initWithLevel:kSentryLevelError];
	SentryException *sentryException = [[SentryException alloc] initWithValue:exception.reason
	type:exception.name];