ID 273783 - Material plugins require an input

Problem summary
Trying to create a Material subclass which has no inputs will cause a crash when it's used.

Customer reported version
nuke.10.5v2

Customer reported platform
windows10

Steps to reproduce
Developer repro steps available in comments

Workaround
Change the min/max inputs to return 1, the default input will be created, and it will work.

Reproduced by support
Nuke 10.5v3 - OSX 10.11.6
Reproduced by developers

Expected behaviour
No crash to occur

Actual behaviour
Crash occurred