Fix null errors

[loongyh]

Managed to come up with a fix for the null errors, inspired from the comment by @Mc-muffin. All chats including group chats transferred over intact, however group chats with members who quit have blank senders. Fixes #22

[npala9520]

@loongyh Still getting error, even after copy pasting your updated watoi.m code.
2020-07-04 15:17:04.454 watoi[691:9900] Loaded 257 chat(s)
2020-07-04 15:17:04.470 watoi[691:9900] Importing messages for chat: 1[number1]-1[number2]@g.us
2020-07-04 15:17:04.470 watoi[691:9900] missing sender
2020-07-04 15:17:04.472 watoi[691:9900] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unacceptable type of value for attribute: property = "memberJID"; desired type = NSString; given type = NSNull; value = .'
*** First throw call stack:
(
0 CoreFoundation 0x00007fff31909d07 __exceptionPreprocess + 250
1 libobjc.A.dylib 0x00007fff6a62d5bf objc_exception_throw + 48
2 CoreData 0x00007fff3135be12 _PFManagedObject_coerceValueForKeyWithDescription + 3234
3 CoreData 0x00007fff31337f48 _sharedIMPL_setvfk_core + 232
4 watoi 0x0000000103d19a39 -[Importer addMissingMember:toChat:asAdmin:] + 313
5 watoi 0x0000000103d1b54d -[Importer importMessages] + 1794
6 watoi 0x0000000103d18a22 main + 346
7 libdyld.dylib 0x00007fff6b7d4cc9 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
zsh: abort build/Release/watoi /Users/[username]/Downloads/msgstore.db

Any thoughts? Thanks--
android WhatsApp 2.20.193.9
iOS WhatsApp 2.20.70

[loongyh]

memberJID has to be String instead of Null.
I no longer have the dev env set-up nor the time thus am unable to look into this.