basic/strv: fix strv_join for first empty argument

basic/strv: fix strv_join for first empty argument

Empty strings were ignored in strv_join, but only if they were at the beginning
of the string. Empty strings after at least one non-empty item were treated
normally.

Previously:
{"x"} → "x"
{"x", ""} → "x"
{"x", "", ""} → "x::"
{""} → ""
{"", ""} → ""
{"", "", ""} → ""
{"", "x"} → "x"
{"", "x", ""} → "x:"
Now:
{"x"} → "x"
{"x", ""} → "x"
{"x", "", ""} → "x::"
{""} → ""
{"", ""} → ":"
{"", "", ""} → "::"
{"", "x"} → ":x"
{"", "x", ""} → ":x:"