Fix nil error in warn logs about incompatible matchers

[grobinson-grafana]

This commit fixes a small bug in the warning logs for incompatible matchers where the error from the UTF-8 parser was logged as nil.

[grobinson-grafana]

Alertmanager server:

alertmanager --config.file=config.yml
ts=2024-01-26T11:18:04.085Z caller=main.go:181 level=info msg="Starting Alertmanager" version="(version=0.26.0, branch=main, revision=1da134aa30c81e656c5156df1499a77d5df92269)"
...
ts=2024-01-26T11:18:04.116Z caller=parse.go:222 level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the old matchers parser as a fallback. To make this input compatible with the new parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue." input="foo=" origin=config err="end of input: expected label value" suggestion="foo=\"\""
ts=2024-01-26T11:18:04.117Z caller=coordinator.go:126 level=info component=configuration msg="Completed loading of configuration file" file=config.yml

amtool:

amtool --alertmanager.url=http://127.0.0.1:9090 alert add foo=
level=warn msg="Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the old matchers parser as a fallback. To make this input compatible with the new parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue." input="foo=" origin=cli err="end of input: expected label value" suggestion="foo=\"\""