Update-AzureRmEventGridSubscription doesn't allow updating subjectBeginsWith/subjectEndsWith to empty strings.

[kalyanaj]

Description

Update-AzureRmEventGridSubscription doesn't allow updating subjectBeginsWith/subjectEndsWith to empty strings.

Script/Steps for Reproduction

[From customer]
When value is not null or empty then it works fine. But when it needs to be set to '' it throws
"Cannot validate argument on parameter 'SubjectBeginsWith'. The argument is null or empty. Provide an argument that is not null or empty, and then try the command again"

Update-AzureRmEventGridSubscription -ResourceGroup $ResourceGroupName -TopicName $EventGridTopicName -EventSubscriptionName $Subscription.name -SubjectEndsWith $Subscription.subjectEndsWith