[nit] NodeCredential should be initialized with nullptr

[erjiaqing]

Problem

We did not init NodeCredential with nullptr, thus is will be a random number, and device controller may crash in real device and cirque virtual test

Summary of Changes

Fix it.

[github-actions]

Size increase report for "esp32-example-build" from e5cb796

File	Section	File	VM
chip-all-clusters-app.elf	.flash.text	20	20

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-pigweed-app.elf and ./pull_artifact/chip-pigweed-app.elf:

sections,vmsize,filesize

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_info,0,234
.debug_str,0,174
.debug_loc,0,130
.debug_line,0,59
.flash.text,20,20
.debug_ranges,0,8
.xt.prop._ZTVN4chip11DeviceLayer37DeviceNetworkProvisioningDelegateImplE,0,-1

[woody-apple]

Allowing hot fix as this is tested and confirmed by CSG.

[github-actions]

Size increase report for "nrfconnect-example-build" from e5cb796

File	Section	File	VM
chip-lock.elf	text	16	16
chip-lighting.elf	text	16	16

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_info,0,245
.debug_str,0,174
.debug_loc,0,118
.debug_line,0,23
.debug_ranges,0,16
text,16,16
.debug_frame,0,-8

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.debug_info,0,245
.debug_str,0,174
.debug_loc,0,122
.debug_line,0,27
.debug_ranges,0,16
text,16,16
.debug_frame,0,-8

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize