Simplify `if' into oneliner, allow spaces in HISTFILE

lib/history.zsh

@@ -1,8 +1,5 @@
 ## Command history configuration
-if [ -z "$HISTFILE" ]; then
-    HISTFILE=$HOME/.zsh_history
-fi
-
+[ -z "$HISTFILE" ] && HISTFILE="$HOME/.zsh_history"
 HISTSIZE=10000
 SAVEHIST=10000