Set default ETag value if vendor returns empty md5 string (#4409)
Set default ETag value if vendor returns empty md5 string (#4409)

The ETag is constructed from md5 atttribute of object attributes
returned by the vendor's Composer. The md5 attribute comes back
as nil for large uploads. Instead the CRC32C should be used.

Refer to https://cloud.google.com/storage/docs/hashes-etags

Fixes #4397

krisis/minio,abperiasamy/minio,krishnasrinivas/minio,jimjag/minio,harshavardhana/minio,NitishT/minio,fwessels/minio,rushenn/minio,abperiasamy/minio,krishnasrinivas/minio,aead/minio,fwessels/minio,aead/minio,donatello/minio,dvstate/minio,NitishT/minio,aead/minio,rushenn/minio,minio/minio,krisis/minio,vadmeste/minio,abperiasamy/minio,jimjag/minio,aead/minio,dvstate/minio,aead/minio,balamurugana/minio,harshavardhana/minio,minio/minio,krishnasrinivas/minio,abperiasamy/minio,aead/minio,harshavardhana/minio,NitishT/minio,aead/minio,dvstate/minio,harshavardhana/minio,balamurugana/minio,rushenn/minio,fwessels/minio,jimjag/minio,harshavardhana/minio,dvstate/minio,donatello/minio,harshavardhana/minio,fwessels/minio,aead/minio,harshavardhana/minio,vadmeste/minio,NitishT/minio,harshavardhana/minio,jimjag/minio,rushenn/minio,krishnasrinivas/minio