Fix error - TypeError: instance of IO needed

What does this MR do?

When introducing changes in #37911 (closed) I haven't thought about the situation when there is a lookup key, but there is not associated Active Session in Redis. It caused loading nil into Marshall and error

TypeError: instance of IO needed
  active_support/core_ext/marshal.rb:6:in `load'
    super(source, proc)
  active_support/core_ext/marshal.rb:6:in `load'
    super(source, proc)
  active_session.rb:137:in `block in active_session_entries'
    Marshal.load(raw_session) # rubocop:disable Security/MarshalLoad
  active_session.rb:136:in `map'
    entry_keys.map do |raw_session|
  active_session.rb:136:in `active_session_entries'
    entry_keys.map do |raw_session|
...
(136 additional frame(s) were not displayed)

TypeError: instance of IO needed

This MR takes care of this problem by removing nils before Marshall call.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #37911 (closed)