It "looks" like you have a hit from an empty search query. If you can find the entry you can just delete it.

If not then you'd have to hack the core a tad, inc/model/sessions/_hitlog.funcs, change this section of code to match ( approx lines 296 -> 308 )

		if( 
			( $param_parts[0] == 'q'
				or $param_parts[0] == 'as_q' 		// Google Advanced Search Query
				or $param_parts[0] == 'query'
				or $param_parts[0] == 'search'
				or $param_parts[0] == 'p'
				or $param_parts[0] == 'kw'
				or $param_parts[0] == 'qs'
				or $param_parts[0] == 'r'
				or $param_parts[0] == 'rdata'				// search.ke.voila.fr
				or $param_parts[0] == 'su'				// suche.web.de
			)
			and !empty( $param_parts[1] )
		)
		{ // found "q" query parameter
			$q = urldecode($param_parts[1]);

¥

If I just ignore it will it go away when it gets pruned

There's always that option :p

¥

Thanks for your help, I think i will wait for it to expire.