add check for 'if x == nil && len(x) > 0'

[dmitris]

PR adds a check for a pattern from a recent PR review - with x being a slice (or map), an erroneous check for x == nil && len(x) > 0, two contradictory conditions that cannot be true at the same time, since the length of a nil slice or map is defined to be 0:

• https://go.dev/tour/moretypes/12 "A nil slice has a length and capacity of 0 and has no underlying array."
• https://go.dev/doc/effective_go#slices "len and cap are legal when applied to the nil slice, and return 0."