[rust-server]: Fix NPE

[mvniekerk]

I had an NPE due to a malformed OAS YAML file. This 3 liner fixes the frailty on this malformed-ness and spews out an error message with the offending parameter.

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[mvniekerk]

CC @frol @farcaller @bjgill @richardwhiuk

[jimschubert]

Is this actually a user-facing error? I feel like someone running the generator would see this message and think generation has failed or skipped something important.

Would you consider removing the error and just inverting your equals check to avoid the NPE?

if( uuidType.equals(param.dataType) ) {

[wing328]

I've filed #5304 with @jimschubert suggestion.