Handle empty cache while using Range or RangeBackwards

[davseby]

After adding this snippet:

deletedCache := New[string, string]()
addToCache(deletedCache, time.Minute, "6", "3", "4")
assert.NotPanics(t, func() {
	deletedCache.Range(func(item *Item[string, string]) bool {
		deletedCache.DeleteAll()
		return true
	})
})

I received a panic as the check part of the loop assumed that the cache wouldn't be empty. A simple len check fixes this.

Closes #174.

[coveralls]

Pull Request Test Coverage Report for Build 15237467069

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 99.711%

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Totals
Change from base Build 14738370205:	0.3%
Covered Lines:	689
Relevant Lines:	691

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💛 - Coveralls