With the release of NuGet version 3.4, we (the NuGet team) tightened the validation of dependencies. We found that NuGet used to take any invalid dependency and turn it into an empty one. That could lead into problems in package resolution or updates.
It turns out that the nexus server is sending the string null to represent an empty dependency, which was not something the version parser could parse, and it turned into an error.
We are going to allow it going forward (3.4.2+), but we recommend rectifying the string to be empty like all other servers do.
Here is the bug in NuGet https://github.com/NuGet/Home/issues/2426
Specifically this is the offending xml snippet: