git-gui: Don't show blame tooltips that we have no data for

If we haven't yet loaded any commit information for a given line but
our tooltip timer fired and tried to draw the tooltip we shouldn't;
there is nothing to show.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/blame.tcl

@@ -879,6 +879,11 @@ method _open_tooltip {cur_w} {
 		set org [lindex $amov_data $lno]
 	}
 
+	if {$dat eq {}} {
+		_hide_tooltip $this
+		return
+	}
+
 	set cmit [lindex $dat 0]
 	set tooltip_commit [list $cmit]