Don't panic when defer a cache that has failed to init

[thomaspoignant]

Description

If you the ffclient.Init method return an error and you don't exit at that moment, the ffclient.Close function can panic.

Example:

	err := ffclient.Init(ffclient.Config{
		PollInterval: 3,
		Retriever: &ffclient.FileRetriever{
			Path: "testdata/flag-config2.toml",
		},
		FileFormat: "toml",
	})
	defer ffclient.Close()

This PR avoids panic while calling the Close method if no cache initialized.

Changes include

• Bugfix (non-breaking change that solves an issue)
• New feature (non-breaking change that adds functionality)
• Breaking changes (change that is not backward-comptible and/or changes current functionality)

Checklist

• I have tested this code
• I have added unit test to cover this code
• I have updated the Readme
• I have followed the contributing guide

[sonarcloud]

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