fix: be defensive when given a falsy context value

[snyamathi]

@redonkulus @BillDorn when given a falsy context value, value.constructor === Array throws a runtime error. We can instead use Array.isArray which is probably a bit neater anyhow.

It's possible that we could also fix this by putting an early return/continue before any such falsy values but given my limited familiarity with this repo I'd rather keep the scope of the change small.

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[snyamathi]

It looks like node 6 failed at the lint step since they're using the spread operator w/in eslint iteslf.

> [email protected] lint /home/travis/build/yahoo/ycb
> eslint .
/home/travis/build/yahoo/ycb/node_modules/eslint/lib/cli-engine/cli-engine.js:257
        ...calculateStatsPerFile(messages)
        ^^^
SyntaxError: Unexpected token ...
    at NativeCompileCache._moduleCompile (/home/travis/build/yahoo/ycb/node_modules/v8-compile-cache/v8-compile-cache.js:242:18)
    at Module._compile (/home/travis/build/yahoo/ycb/node_modules/v8-compile-cache/v8-compile-cache.js:186:36)
    at Object.Module._extensions..js (module.js:586:10)
    at Module.load (module.js:494:32)
    at tryModuleLoad (module.js:453:12)
    at Function.Module._load (module.js:445:3)
    at Module.require (module.js:504:17)
    at require (/home/travis/build/yahoo/ycb/node_modules/v8-compile-cache/v8-compile-cache.js:161:20)
    at Object.<anonymous> (/home/travis/build/yahoo/ycb/node_modules/eslint/lib/cli-engine/index.js:3:23)
    at Module._compile (/home/travis/build/yahoo/ycb/node_modules/v8-compile-cache/v8-compile-cache.js:194:30)

[snyamathi]

eslint/eslint#11944 (comment)

[redonkulus]

Node 8 and 10 are breaking too