Add test for closing a nil smtpclient #62

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	52s
Dependency review	13s
Go vulnerabilities check	23s
golangci-lint (1.23)	27s
REUSE Compliance Check	15s
Test with SonarQube review (ubuntu-latest / 1.23)	2m 20s
Test (macos-latest / 1.19)	30s
Test (macos-latest / 1.20)	36s
Test (macos-latest / 1.21)	21s
Test (macos-latest / 1.22)	20s
Test (macos-latest / 1.23)	22s
Test (ubuntu-latest / 1.19)	28s
Test (ubuntu-latest / 1.20)	26s
Test (ubuntu-latest / 1.21)	28s
Test (ubuntu-latest / 1.22)	24s
Test (ubuntu-latest / 1.23)	24s
Test (windows-latest / 1.19)	1m 9s
Test (windows-latest / 1.20)	1m 59s
Test (windows-latest / 1.21)	1m 8s
Test (windows-latest / 1.22)	1m 55s
Test (windows-latest / 1.23)	53s
Test on FreeBSD 13.4'	2m 28s
Test on FreeBSD 14.0	2m 32s
Test on FreeBSD 14.1	2m 16s
	23m 9s

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