[chore]: Improve test coverage of expvar extension (#7484)

## Which problem is this PR solving?
- Improves Code coverage of
`cmd/jaeger/internal/extension/expvar/extension.go`

## Description of the changes
- removes nested if which will reduce amount of untested code.

## How was this change tested?
- 

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [x] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `npm run lint` and `npm run test`

---------

Signed-off-by: anmol7344 <[email protected]>
Signed-off-by: Anmol <[email protected]>
Co-authored-by: graphite-app[bot] <96075541+graphite-app[bot]@users.noreply.github.com>

Author: AnmolxSingh

cmd/jaeger/internal/extension/expvar/extension.go

@@ -63,11 +63,12 @@ func (c *expvarExtension) Start(ctx context.Context, host component.Host) error
 }
 
 func (c *expvarExtension) Shutdown(ctx context.Context) error {
-	if c.server != nil {
-		if err := c.server.Shutdown(ctx); err != nil {
-			return fmt.Errorf("error shutting down expvar server: %w", err)
-		}
-		c.shutdownWG.Wait()
+	if c.server == nil {
+		return nil
 	}
-	return nil
+
+	err := c.server.Shutdown(ctx)
+	c.shutdownWG.Wait()
+
+	return err
 }