Fix migration bugs

[nporter-adbe]

Description

• Fixes a bug where the Identity ECID wasn't migrated from v4 & v5
• Fixes a bug where some lifecycle data (dates) weren't migrated properly
• Fixes a bug in AnyCodable when attempting to encode nil

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #594 (7bfb205) into dev-v3.1.0 (84f7c9c) will increase coverage by 0.08%.
The diff coverage is 94.12%.

@@              Coverage Diff               @@
##           dev-v3.1.0     #594      +/-   ##
==============================================
+ Coverage       85.27%   85.34%   +0.08%     
==============================================
  Files             102      102              
  Lines            4541     4544       +3     
==============================================
+ Hits             3872     3878       +6     
+ Misses            669      666       -3