Fix bug with iteration of empty character class

Check that there is at least one character class before applying the
regex nodes.

When compiling the 'conj' regex node, it expects to be able to shift
off at least one child node and iterates past the end as a result.

Fixes [Rakudo Issue #1622](rakudo/rakudo#1622)

src/QRegex/P6Regex/Actions.nqp

@@ -765,8 +765,8 @@ class QRegex::P6Regex::Actions is HLL::Actions {
             @alts.push(QAST::Regex.new( $str, :rxtype<enumcharlist>, :node($/), :negate( $<sign> eq '-' ),
                                         :subtype($RXm ?? 'ignoremark' !! '') ))
                 if nqp::chars($str);
-            $qast := +@alts == 1 ?? @alts[0] !!
-                $<sign> eq '-' ??
+            $qast := ( my $num := +@alts ) == 1 ?? @alts[0] !!
+                0 < $num && $<sign> eq '-' ??
                     QAST::Regex.new( :rxtype<concat>, :node($/), :negate(1),
                         QAST::Regex.new( :rxtype<conj>, :subtype<zerowidth>, |@alts ),
                         QAST::Regex.new( :rxtype<cclass>, :name<.> ) ) !!