'main': simple parameter expansions at command word: Don't use an undefined value.
'main': simple parameter expansions at command word: Don't use an undefined value.

Causes such expansions to be highlighted as [unknown-token] rather than as default (absent from $region_highlight).

Fixes #337.

danielshahaf/zsh-syntax-highlighting-debian,danielshahaf/zsh-syntax-highlighting-debian,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,phy1729/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,joshuarubin/dotfiles,psprint/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,m0vie/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,stuha/wbr-zsh,danielshahaf/zsh-syntax-highlighting