fixes crash for react native instrumenting fetch

[mxs]

This PR fixes a crash in @sentry/react-native v1.3.3 on startup, caused by assuming global.document.createElement exists (hence assuming web context).

This PR just makes sure global.document.createElement is a function before actually calling it.

• React native uses JavascriptCore and Hermes, global.document in those environments only contains addEventListener and removeEventListener and nothing else.
• In the following commit in @sentry/react-native, line 46 of sdk.ts, fetch:false was removed. fetch:false prevented the crash from happening by bypassing the fetch instrumentation. Hence the mentioned commit triggered an existing bug within sentry-javascript. This bug appeared for us when we upgraded @sentry/react-native from v1.2.1 to v1.3.3

[kamilogorek]

Thanks!