Have OpenGL2 "map $lightmap" check for NULL tr.lightmaps too

Zero length lightmap lump will have NULL tr.lightmaps.

OpenGL1 already has this check, because r_vertexLight 1
would crash Team Arena. OpenGL2 does not disable loading lightmaps
when r_vertexLight is 1 though, so it does not have that issue.

code/renderergl2/tr_shader.c

@@ -639,7 +639,7 @@ static qboolean ParseStage( shaderStage_t *stage, char **text )
 			else if ( !Q_stricmp( token, "$lightmap" ) )
 			{
 				stage->bundle[0].isLightmap = qtrue;
-				if ( shader.lightmapIndex < 0 ) {
+				if ( shader.lightmapIndex < 0 || !tr.lightmaps ) {
 					stage->bundle[0].image[0] = tr.whiteImage;
 				} else {
 					stage->bundle[0].image[0] = tr.lightmaps[shader.lightmapIndex];