Behavior of .add code used on a non-existent key

[muzimuzhi]

Brief outline of the bug

When used on a non-existent key, .add code uses \pgfkeyssetvalue{<full key>}{#1}, not empty, as the initial key body. Is this behavior on purpose/relied on by other codes?

It was introduced in acf16c2 (worked on bug #250, 2013-07-15), in resolving #249. Before acf16c2, empty was used as the initial key body.

The pgfmanual only says .add code is one of handlers to modify existing keys.

pgf/doc/generic/pgf/pgfmanual-en-pgfkeys.tex

Lines 1082 to 1089 in 6e5fd71

	There are also handlers for modifying existing keys.
	\begin{handler}{{.add code}|=|\marg{prefix code}\marg{append code}}
	This handler adds code to an existing key. The \meta{prefix code} is added
	to the code stored in \meta{key}|/.@cmd| at the beginning, the \meta{append
	code} is added to this code at the end. Either can be empty. The argument
	list of \meta{code} cannot be changed using this handler. Note that both
	\meta{prefix code} and \meta{append code} may contain parameters like |#2|.

Minimal working example (MWE)

\documentclass{article}
\usepackage{pgfkeys}

\pgfkeys{
  % executes \@namedef{pgfk@/a/.@cmd}#1\pgfeov{}
  a/.code={},
  % executes \@namedef{pgfk@/a/.@cmd}#1\pgfeov{<pre a><post a>}
  a/.add code={<pre a>}{<post a>},
  % executes \@namedef{pgfk@/b/.@cmd}#1\pgfeov{<pre b>\pgfkeyssetvalue{/b}{#1}<post b>}
  % why the "\pgfkeyssetvalue{/b}{#1}" part?
  b/.add code={<pre b>}{<post b>},
  a/.show code,
  b/.show code
}

\begin{document}
\end{document}