[v9] Passing `undefined` to SDK init options should behave like not setting the options

[Lms24]

Description

Currently, we SDK behaviour differs (slightly) if we in higher level SDKs or users in general pass undefined to certain SDK init options instead of not setting them at all.

Examples:

• defaultIntegrations: undefined causes no default integrations to be installed while not setting defaultIntegrations means default integrations get installed - fixed in fix(browser): Initialize default integration if defaultIntegrations: undefined #13261
• tracesSampleRate: undefined makes the hasTracingEnabled return true, while not setting it (assuming no other tracing options are set) makes hasTracingEnabled return false.

Ideally, we just call dropUndefinedKeys on the user-passed options and move on. However, for this to work we need to change the implementation of hasTracingEnabled to not differentiate between the two possibilities. Also this questions the validity of hasTracingEnabled entirely because even if no sample rates are set at all, "Tracing without Performance" should be enabled.

[Lms24]

The good news is that this seems mostly to be a browser issue as the Node SDK handles user option merging a bit differently.