FileSystem#getFileChecksum may accept length parameter and 0 is a valid value. Currently it will return null when length is 0, in the following code block:
The comment says "we should never get here since the validity was checked" but it does. As we're using the MD5-MD5-X approach, and EMPTY--CONTENT actually is a valid case in which the MD5 value is d41d8cd98f00b204e9800998ecf8427e, so suggest we return a reasonable value other than null. At least some useful information in the returned value can be seen, like values from block checksum header.