Cellular: Fix ATHandler string handling

[AriParkkila]

Description

Summary of change

Fix ATHandler::read_string/skip_param/read_hex_string to handle sequence "".
Fix ATHandler::write_hex_string to have quotation marks.

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@mirelachirica

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Release Notes

Summary of changes

Impact of changes

Migration actions required

[AriParkkila]

what about useQuatations parameter as for read_string(...)

@mirelachirica That's likely needed at some point but not right now, and since that'd be an API change it's better to fix the default to conform to 3GPP TS 27.007.

[ciarmcom]

@AriParkkila, thank you for your changes.
@mirelachirica @ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts