jenkins: guard potentially unset parameter

[richardlau]

Guard against the NODEJS_VERSION parameter not being set, which
results in parameters['NODEJS_VERSION'] being null.

Refs: https://ci.nodejs.org/job/node-compile-windows/46654/console

08:02:32 println "Node.js version: ${new String(parameters['NODEJS_VERSION'])}"
...
08:02:32 FATAL: Ambiguous method overloading for method java.lang.String#<init>.
08:02:32 Cannot resolve which method to invoke for [null] due to overlapping prototypes between:
08:02:32 	[class [B]
08:02:32 	[class [C]
08:02:32 	[class java.lang.String]
08:02:32 groovy.lang.GroovyRuntimeException: Ambiguous method overloading for method java.lang.String#<init>.
08:02:32 Cannot resolve which method to invoke for [null] due to overlapping prototypes between:
08:02:32 	[class [B]
08:02:32 	[class [C]
08:02:32 	[class java.lang.String]
08:02:32 	at groovy.lang.MetaClassImpl.doChooseMostSpecificParams(MetaClassImpl.java:3251)

[mhdawson]

LGTM