Fixes nil pointer on workgroup tags

[veqryn]

This PR fixes the following panic:
If you create an athena Config with a named athena workgroup, with the default workgroup tags, it will give a nil pointer panic when it tries to create the workgroup (and also if your aws permissions are incorrect).

This is because when QueryContext goes to get the current workgroup, even if the workgroup has the default tags, it will initialize a new workgroup with nil for tags, resulting in a nil pointer error when the code later tries to get the tags.

runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xf8b325]

goroutine 1 [running]:
github.com/uber/athenadriver/go.(*WGTags).Get(...)
        /go/src/vendor/github.com/uber/athenadriver/go/wgtag.go:44
github.com/uber/athenadriver/go.(*Workgroup).CreateWGRemotely(0xc0009bd028, {0x1a2cbc8, 0xc0001b4510})
        /go/src/vendor/github.com/uber/athenadriver/go/wg.go:81 +0x25
github.com/uber/athenadriver/go.(*Connection).QueryContext(0xc0008046a0, {0x19fd430, 0x2520360}, {0x16d43f6?, 0xa?}, {0x2520360, 0x0, 0x30?})
        /go/src/vendor/github.com/uber/athenadriver/go/connection.go:279 +0xbc5
database/sql.ctxDriverQuery({0x19fd430?, 0x2520360?}, {0x7fffb83282b8?, 0xc0008046a0?}, {0x0?, 0x0?}, {0x16d43f6?, 0x0?}, {0x2520360, 0x0, ...})
        /usr/local/go/src/database/sql/ctxutil.go:48 +0xd7
database/sql.(*DB).queryDC.func1()
        /usr/local/go/src/database/sql/sql.go:1778 +0x159
database/sql.withLock({0x19f3cb0, 0xc00081a500}, 0xc00062d428)
        /usr/local/go/src/database/sql/sql.go:3566 +0x71
database/sql.(*DB).queryDC(0x1?, {0x19fd430, 0x2520360}, {0x0, 0x0}, 0xc00081a500, 0xc000806670, {0x16d43f6, 0xa}, {0x0, ...})
        /usr/local/go/src/database/sql/sql.go:1773 +0x1de
database/sql.(*DB).query(0xc0001bf2b0, {0x19fd430, 0x2520360}, {0x16d43f6, 0xa}, {0x0, 0x0, 0x0}, 0x88?)
        /usr/local/go/src/database/sql/sql.go:1756 +0xfc
database/sql.(*DB).QueryContext.func1(0x0?)
        /usr/local/go/src/database/sql/sql.go:1734 +0x4f
database/sql.(*DB).retry(0xc000804640?, 0xc0009bd608)
        /usr/local/go/src/database/sql/sql.go:1568 +0x42
database/sql.(*DB).QueryContext(0x9?, {0x19fd430?, 0x2520360?}, {0x16d43f6?, 0x0?}, {0x0?, 0xc00062d6c8?, 0x4123a5?})
        /usr/local/go/src/database/sql/sql.go:1733 +0xc5
database/sql.(*DB).QueryRowContext(...)
        /usr/local/go/src/database/sql/sql.go:1834
database/sql.(*DB).QueryRow(0x16d3100?, {0x16d43f6?, 0xc000146500?}, {0x0?, 0x2?, 0x6?})
        /usr/local/go/src/database/sql/sql.go:1848 +0x45

Reproduce-able setup:
(Just make sure the workgroup doesn't exist yet or you have bad credentials.)

	athenaCfg := athenadrv.NewNoOpsConfig()
	athenaCfg.SetResultPollIntervalSeconds(athenadrv.PoolInterval)
	athenaCfg.SetWGRemoteCreationAllowed(true)
	athenaCfg.SetDB(athenaDatabase)

	err := athenaCfg.SetRegion(awsRegion)
	if err != nil {
		return nil, fmt.Errorf("unable to set athena region: %w", err)
	}

	err = athenaCfg.SetOutputBucket(fmt.Sprintf("s3://%s/%s", athenaResultsBucket, athenaResultsPath))
	if err != nil {
		return nil, fmt.Errorf("unable to set athena output bucket: %w", err)
	}

	err = athenaCfg.SetWorkGroup(athenadrv.NewDefaultWG(athenaWorkgroup, &athenav1.WorkGroupConfiguration{
		EnforceWorkGroupConfiguration:   aws.Bool(false),
		PublishCloudWatchMetricsEnabled: aws.Bool(true),
	}, nil))
	if err != nil {
		return nil, fmt.Errorf("unable to set athena workgroup: %w", err)
	}

	athenaDB, err := sql.Open(athenadrv.DriverName, athenaCfg.Stringify())

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