DBAL-1167 #814 allow serverVersion to be explicitly unspecified (null)

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of BreiteSeite:

Url: #814

Message:

Hi,

this PR allows serverVersionto be nullable.

We use doctrine/dbal in Integration-Tests and to prevent the unit-tests from connecting to a database, we specify serverVersion with something made-up (like 5.6).

However, i'm not comfortable set serverVersion to a made-up server-version to prevent connections from being made, when there even isn't a server to connect to. With this PR merged, we could specify serverVersion to be null instead of something like 5.6 and still prevent connections from being made. null is a valid return value for getDatabasePlatformVersion().

[Ocramius]

Handled in #2671

[github-actions]

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