Minor optimisation to mcr20a-rf-driver code

[DavidLin1577]

Summary of changes

This is a minor optimisation to the mcr20a-rf-driver code:

Note that function parameter is pointer. The code is more readable when 'NULL' instead of using '0'.

Impact of changes

Migration actions required

Documentation

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Pull request type

[*] 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)
[*] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[0xc0170]

@artokin Isn't this code coming from upstream repo, therefore should be fixed there first?

[artokin]

@0xc0170 we can backport these changes there to keep repositories in sync.
cc: @JarkkoPaso

[0xc0170]

CI started

[0xc0170]

@DavidLin1577 please include a header file for NULL declaration, see Travis errors

./components/802.15.4_RF/mcr20a-rf-driver/source/MCR20Drv.c:409:44: error: 'NULL' undeclared (first use in this function)
     if ((numOfBytes == 0) || (byteArray == NULL)) {

[mbed-ci]

Test run: FAILED

Summary: 3 of 4 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-IAR
• jenkins-ci/mbed-os-ci_build-GCC_ARM
• jenkins-ci/mbed-os-ci_build-ARM

Pull request has been modified.

[DavidLin1577]

@DavidLin1577 please include a header file for NULL declaration, see Travis errors

./components/802.15.4_RF/mcr20a-rf-driver/source/MCR20Drv.c:409:44: error: 'NULL' undeclared (first use in this function)
     if ((numOfBytes == 0) || (byteArray == NULL)) {

@0xc0170 thank you so much. I had changed it as your suggestion.

[0xc0170]

Ci started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts