NPE restoring cached script with optimistic types disabled

A NullPointerException occurs when installing scripts from persistent cache with optimistic types disabled. When running the script below twice:

$ java -jar dist/nashorn.jar --optimistic-types=false -pcc ./test/script/basic/JDK-8048079_2a.js
parsed and compiled ok prototype.js
$ java -jar dist/nashorn.jar --optimistic-types=false -pcc ./test/script/basic/JDK-8048079_2a.js
Exception in thread "main" java.lang.NullPointerException
at jdk.nashorn.internal.runtime.Context.install(Context.java:1249)
at jdk.nashorn.internal.runtime.Context.compile(Context.java:1192)
at jdk.nashorn.internal.runtime.Context.compileScript(Context.java:1123)
at jdk.nashorn.internal.runtime.Context.evaluateSource(Context.java:1089)
at jdk.nashorn.internal.runtime.Context.load(Context.java:773)
at jdk.nashorn.internal.objects.Global.load(Global.java:1000)
at jdk.nashorn.internal.scripts.Script$JDK_8048079_2a.:program(./test/script/basic/JDK-8048079_2a.js:34)
at jdk.nashorn.internal.runtime.ScriptFunctionData.invoke(ScriptFunctionData.java:636)
at jdk.nashorn.internal.runtime.ScriptFunction.invoke(ScriptFunction.java:229)
at jdk.nashorn.internal.runtime.ScriptRuntime.apply(ScriptRuntime.java:387)
at jdk.nashorn.tools.Shell.apply(Shell.java:394)
at jdk.nashorn.tools.Shell.runScripts(Shell.java:323)
at jdk.nashorn.tools.Shell.run(Shell.java:169)
at jdk.nashorn.tools.Shell.main(Shell.java:133)
at jdk.nashorn.tools.Shell.main(Shell.java:112)

Obviously a function in the constants array may not have an initializer when lazy compilation is enabled, so we need to add a null check right there.