Always execute preview command if {q} is in the template

Even when {q} is empty. Because, why not?

While this can be seen as a breaking change, there is an easy workaround
to keep the old behavior.

    # This will show // even when the query is empty
    : | fzf --preview 'echo /{q}/'

    # But if you don't want it,
    : | fzf --preview '[ -n {q} ] || exit; echo /{q}/'

Close #2759

src/terminal.go

@@ -2096,7 +2096,7 @@ func (t *Terminal) currentItem() *Item {
 func (t *Terminal) buildPlusList(template string, forcePlus bool) (bool, []*Item) {
 	current := t.currentItem()
 	slot, plus, query := hasPreviewFlags(template)
-	if !(!slot || query && len(t.input) > 0 || (forcePlus || plus) && len(t.selected) > 0) {
+	if !(!slot || query || (forcePlus || plus) && len(t.selected) > 0) {
 		return current != nil, []*Item{current, current}
 	}