feat: allow empty value for /pgf/arrow keys/fill

doc/generic/pgf/pgfmanual-en-tikz-arrows.tex

@@ -796,7 +796,7 @@ \subsubsection{Coloring}
     filled.
 \end{key}
 
-\begin{key}{/pgf/arrow keys/fill=\meta{color or |none|}}
+\begin{key}{/pgf/arrow keys/fill=\meta{color or |none| or empty}}
     Use this key to explicitly set the color used for filling the arrow tips.
     This color can be different from the color used to draw (stroke) the arrow
     tip:
@@ -818,6 +818,11 @@ \subsubsection{Coloring}
 }
 \end{codeexample}
     %
+    If the argument is left empty or omitted altogether, no explicit fill color
+    will be set, meaning that the fill color will be determined by whatever was
+    used last as a fill color.  In case that the ``current'' fill color is
+    intended, it is better to explicitly use |fill=.|.
+
     Note that such ``open'' arrow tips are a bit difficult to draw in some
     case: The problem is that the line must be shortened by just the right
     amount so that it ends exactly on the back end of the arrow tip. In some

tex/generic/pgf/basiclayer/pgfcorearrows.code.tex

@@ -510,12 +510,13 @@
   fill/.code={%
     \def\pgf@temp{#1}%
     \ifx\pgf@temp\pgf@nonetext%
-      \pgfarrowsaddtooptions{\pgfarrowopentrue}
+      \pgfarrowsaddtooptions{\pgfarrowopentrue}%
+    \else\ifx\pgf@temp\pgfkeysnovalue@text%
+      \pgfarrowsaddtooptions{\pgfarrowopenfalse}%
     \else
-      \pgfarrowsaddtooptions{\pgfarrowopenfalse\def\pgf@arrows@fill@color{#1}}
-    \fi
+      \pgfarrowsaddtooptions{\pgfarrowopenfalse\def\pgf@arrows@fill@color{#1}}%
+    \fi\fi
   },
-  fill/.value required,
   open/.style={fill=none},
   .unknown/.code={
     \expandafter\pgfutil@in@\expandafter!\expandafter{\pgfkeyscurrentname}%