remove unnecessary nil checks before calling len() on slices

[knbr13]

this PR removes unnecessary nil checks before calling len() on slices.,

var slice []byte // equal to nil, default value for a slice
if slice != nil && len(slice) > 1 {}
// is the same as
if len(slice) > 1 {}

because len(slice) where slice == nil is always equal to 0,

staticcheck will highlight this optimization with the following message:

log.go:497:5: should omit nil check; len() for []byte is defined as zero (S1009)