Pod: allow empty config value

Previously a Pod block such as

  =begin pod :config('')
  =end pod

or

  =begin pod :config[]
  =end pod

would throw a compiler error (coercing NQPMu to Str). This is because,
first, an empty quoted string was treated like nothing when compiling
the list of values of the :config key above, and second, an empty value
list was unexpected on the caller site.

This commit makes empty quoted strings recognized and empty value lists
supported, so that

  =begin pod :x('') :y( )
  =end pod

parses successfully and gives

  dd $=pod.head.config
  #= Hash %!config = {:x(""), :y($[])}

Fixes #3098 and closes #3101

src/Perl6/Pod.nqp

@@ -486,7 +486,7 @@ class Perl6::Pod {
                 $quoted   := nqp::null();
             }
 
-            if nqp::chars($quoted) {
+            if nqp::chars($quote) || nqp::chars($quoted) {
                 $word := nqp::concat($word, $quoted);
             }
             elsif nqp::chars($unquoted) {
@@ -498,7 +498,7 @@ class Perl6::Pod {
                 @pieces.push($delim) if ($keep eq 'delimiters');
                 $word := '';
             }
-            if !nqp::chars($line) && nqp::chars($word) {
+            if !nqp::chars($line) && (nqp::chars($quote) || nqp::chars($word)) {
                 @pieces.push($word);
                 $word := '';
             }
@@ -526,14 +526,15 @@ class Perl6::Pod {
         # break into an array
         my @arr := string2array($st);
 
-        if nqp::elems(@arr) > 1 {
-            return serialize_object('Array', |@arr).compile_time_value;
-        }
-        else {
+        if nqp::elems(@arr) == 1 {
             # convert a single-element list to a single value
             my $val := @arr[0];
             return $val;
         }
+        else {
+            # 0 or 2 or more elements are an array
+            return serialize_object('Array', |@arr).compile_time_value;
+        }
     }
 
     sub make-config-hash($st) {