fix(Spanner): default creator role is empty str

[vishwarajanand]

We need to migrate default creator role to empty string instead of a null string to avoid failures in System\SessionTest::testCacheSessionPool where creator_role is null (not empty string) and emulator is rejecting it:

google-cloud-php/Spanner/src/Session/CacheSessionPool.php

Line 695 in 06e3f61

'creator_role' => isset($this->config['databaseRole']) ? $this->config['databaseRole'] : null

This change will bring spanner's creator roles in line with other language libraries, such as Go and with autogenerated changes:

google-cloud-php/Spanner/src/V1/Session.php

Line 54 in fb87e51

private $creator_role = '';

Note: unfortunately, this will be a BC breaking change.

[ajupazhamayil]

As per the discussion, no defaults mentioned in the proto file here https://github.com/googleapis/googleapis/blob/master/google/spanner/v1/spanner.proto#L362. Seems like the string type in proto automatically assumes empty string as defaults in the gapic files here

google-cloud-php/Spanner/src/V1/Session.php

Line 54 in fb87e51

private $creator_role = '';