Fix buildkitd panic when frontend input is nil.

[sipsma]

See integ test case for repro, it causes a server-side panic.

Probably a bunch of possible fixes on different levels but opted for the "lowest-level" one by handling this case in NewDefinitionOp for now; reduces what was a panic to an err return there. Let me know if it's preferable to handle this elsewhere and make NewDefinitionOp return nil, nil in this case or something like that @tonistiigi

[tonistiigi]

Can this appear in some conditions where functions are chained or does user need to explicitly send nil value?

[sipsma]

Can this appear in some conditions where functions are chained or does user need to explicitly send nil value?

@tonistiigi It seems like for the bridgeClient implementation of Inputs the client has to pretty explicitly just pass nil based on the call paths I looked at.

However I guess looking at the grpcClient implementation of Inputs (not covered by the repro we encountered) maybe there are more complicated situations a similar error could be encountered? But if so, I think the fix to handle this in NewDefinitionOp should handle that better now too.