Add Null check when accessing system service

[fractalwrench]

Goal

context.getSystemService can return a nullable value. #365 addressed a specific instance where this could cause a NullPointerException - this PR addresses the other scenarios where this could potentially occur within the SDK, and defaults to a sensible behaviour.

Changeset

Added null checks within:

• DefaultDelivery: allow delivery to proceed if we can't check the connection beforehand
• Client connectivity receiver: no-op if we receive a connectivity change event and the ConnectivityManager service is null
• DeviceData: explicitly return null if the ConnectivityManager is null, rather than triggering then catching an NPE

Tests

Ran unit tests and mazerunner locally

Review

For the submitter, initial self-review:

• Commented on code changes inline explain the reasoning behind the approach
• Reviewed the test cases added for completeness and possible points for discussion
• A changelog entry was added for the goal of this pull request
• Check the scope of the changeset - is everything in the diff required for the pull request?
• This pull request is ready for:
  • Initial review of the intended approach, not yet feature complete
  • Structural review of the classes, functions, and properties modified
  • Final review

For the pull request reviewer(s), this changeset has been reviewed for:

• Consistency across platforms for structures or concepts added or modified
• Consistency between the changeset and the goal stated above
• Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
• Usage friction - is the proposed change in usage cumbersome or complicated?
• Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
• Concurrency concerns - if components are accessed asynchronously, what issues will arise
• Thoroughness of added tests and any missing edge cases
• Idiomatic use of the language

[fractalwrench]

Fixes #363

[simonbowring]

LGTM