Ruby/Shopify `slice` returns nil (rendered as an empty string or array) when
the begin offset falls outside the negative range or when the length is
negative. liquidjs forwarded the adjusted indices straight to
Array/String.prototype.slice, whose own negative-index handling produced
non-empty, incorrect output:
{{ "hello" | slice: -10, 2 }} => "he" (expected "")
{{ "Liquid" | slice: 1, -2 }} => "iqui" (expected "")
Guard the adjusted begin and the length before slicing.
The `modulo` filter used JavaScript's `%` (truncated remainder, sign
follows the dividend). Shopify/Ruby Liquid uses floored modulo, where the
result takes the sign of the divisor. Since liquidjs advertises Shopify
compatibility, negative operands produced the wrong sign.
Use `((v % arg) + arg) % arg` to match Ruby's `%`. Positive-operand
results are unchanged.
