shell: Fix warning for SHELL_SUBCMD_SET_END

[tpambor]

The macro SHELL_SUBCMD_SET_END was defined as {NULL}, which triggers a -Wmissing-field-initializers warning

shell.c:100:5: warning: missing field 'help' initializer [-Wmissing-field-initializers]
  100 |     SHELL_SUBCMD_SET_END
      |     ^
zephyr/include/zephyr/shell/shell.h:513:35: note: expanded from macro 'SHELL_SUBCMD_SET_END'
  513 | #define SHELL_SUBCMD_SET_END {NULL}

Replacing {NULL} with {0} silences this warning.

Usage of {0} is also a common pattern across the zephyr codebase.

$ grep -R "{0}" . | wc -l
1756

$ grep -R "{NULL}" . | wc -l
14

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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