contrib/gorm.io/gorm.v1: do not panic on open

[bendiknesbo]

If passing a nil *gorm.Config to Open, we should not panic, but instead pass nothing on to gorm.

Fixes #2559

What does this PR do?

This PR fixes a panic when calling Open with a nil-config.
Calling the original gorm library with a nil-config is valid (although no-op),
so we should mimic that behavior in dd-trace-go.

Motivation

Fixes #2559

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!