acc: Allow nullable cdr extra fields

[cruzccl]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue acc: set cdr_extra values to NULL #1378

Description

• allow custom cdr fields defined with cdr_extra modparam to be saved
  as null. Before this change, cdr_extra variables not defined in config
  operation (or set to $null) were saved as empty string (''). This commit makes
  possible to save them as NULL value.

• new modparam to make this behaviour configurable: cdr_extra_nullable.
  Set it to 1 to enable this new behaviour (default value: 0)

[miconda]

Thanks!