xl: Heal empty parts (#7860)

posix.VerifyFile() doesn't know how to check if a file
is corrupted if that file is empty. We do have the part
size in xl.json so we pass it to VerifyFile to return
an error so healing empty parts can work properly.

cmd/xl-v1-healing.go

@@ -195,6 +195,9 @@ func shouldHealObjectOnDisk(xlErr, dataErr error, meta xlMetaV1, quorumModTime t
 		if dataErr == errFileNotFound {
 			return true
 		}
+		if dataErr == errFileUnexpectedSize {
+			return true
+		}
 		if _, ok := dataErr.(HashMismatchError); ok {
 			return true
 		}