in_node_exporter_metrics: textfile: Plug unhandled NULL case

[cosmo0920]

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Enter [N/A] in the box, if an item is not applicable to your change.

Testing
Before we can approve your change; please submit the following in a comment:

• Example configuration file for the change

[SERVICE]
    flush           1
    log_level       debug

[INPUT]
    name            node_exporter_metrics
    tag             fluentbit.metrics.${HOSTNAME}
    scrape_interval 2
    collector.textfile.path /path/to/prom_payloads/prom_metrics/*/*.prom

    metrics textfile

[OUTPUT]
    name stdout
    Match *

• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

==511818== 
==511818== HEAP SUMMARY:
==511818==     in use at exit: 95,170 bytes in 702 blocks
==511818==   total heap usage: 16,152 allocs, 15,450 frees, 50,327,111 bytes allocated
==511818== 
==511818== LEAK SUMMARY:
==511818==    definitely lost: 0 bytes in 0 blocks
==511818==    indirectly lost: 0 bytes in 0 blocks
==511818==      possibly lost: 0 bytes in 0 blocks
==511818==    still reachable: 95,170 bytes in 702 blocks
==511818==         suppressed: 0 bytes in 0 blocks
==511818== Reachable blocks (those to which a pointer was found) are not shown.
==511818== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==511818== 
==511818== For lists of detected and suppressed errors, rerun with: -s
==511818== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.