Fix for checkboxes with an initial value of undefined

[zachintosh]

v2.1.1 introduced a bug with checkboxes where they require an initial value in order to work. This is reproducible and is fixed by back-pinning to v2.1.0. Please view this codesandbox:

The getValueForCheckbox function in Formik.tsx was updated to handle checkbox values better, but it introduced a situation where a checkbox with an undefined value would throw an error, since getValueForCheckbox was assuming it was an array.

Stack Trace:

Uncaught TypeError: Cannot read property 'indexOf' of undefined
    at getValueForCheckbox (formik.cjs.development.js:1216)
    at eval (formik.cjs.development.js:735)
    at eval (formik.cjs.development.js:750)
	...

Here is the line where it breaks:

formik/packages/formik/src/Formik.tsx

Line 1148 in fee503e

const index = currentValue.indexOf(valueProp);

The changes in this PR just shuffle the logic a tiny bit to guarantee this check does not occur unless currentValue is an array.

resolves #2184

[vercel]

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