internal/provider: fix crash setting overrideRegion (#44860)

The step to set an override region was previously missing a check for `nil` region values, potentially triggering a crash due to a nil interface conversion.

Author: jar-b

.changelog/44860.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+provider: Fix crash when setting override region during provider initialization
+```

internal/provider/sdkv2/provider.go

@@ -744,7 +744,7 @@ func (p *sdkProvider) initialize(ctx context.Context) (map[string]conns.ServiceP
 					var overrideRegion string
 
 					if isRegionOverrideEnabled && getAttribute != nil {
-						if region, ok := getAttribute(names.AttrRegion); ok {
+						if region, ok := getAttribute(names.AttrRegion); ok && region != nil {
 							overrideRegion = region.(string)
 						}
 					}