fix: validating undefined connection strings

[hallvictoria]

The extension obtains the connection string by getting the corresponding environment variable / App Setting. If the connection string variable name is not an app setting, the connection property is set to None. When creating a client type and the connection is None, the azure-storage-blob sdk throws an error saying: AttributeError("'NoneType' object has no attribute 'rstrip'").

This is not a clear error, so this adds validation on the connection string in the extension side. If the connection is None, an error will be thrown saying ValueError('Storage account connection string NAME does not exist. Please make sure that it is a defined App Setting.')

The purpose of this PR is not to perform validation on the connection string object. The sdk already does this. The goal of this is to improve the error in the specific case where the connection string is formatted correctly but does not have corresponding value set.

• If the provided connection variable exists but its corresponding connection string value is invalid, the sdk will throw an error saying ValueError('Connection string missing required connection details.')
• If the provided connection string is invalid (eg. contains invalid characters, empty, or whitespace), the sdk will throw an error saying ValueError: Connection string is either blank or malformed..