Avoid loading terrain minimap images when not specified by WML

Otherwise we build a bogus "terrain/.png" file path and produce spurious
`error display: could not open image 'terrain/.png'` messages in stderr.

Although this is primarily an issue found with certain special overlay
terrains such as ^Xo (Impassable Overlay), this fix covers base terrains
as well.

src/minimap.cpp

@@ -137,14 +137,14 @@ surface getMinimap(int w, int h, const gamemap &map, const team *vw, const std::
 						need_highlighting = true;
 					}
 
-					if(i == cache->end()) {
+					if(i == cache->end() && !terrain_info.minimap_image().empty()) {
 						std::string base_file =
 								"terrain/" + terrain_info.minimap_image() + ".png";
 						surface tile = get_image(base_file,image::HEXED);
 
 						//Compose images of base and overlay if necessary
 						// NOTE we also skip overlay when base is missing (to avoid hiding the error)
-						if(tile != NULL && tdata.get_terrain_info(terrain).is_combined()) {
+						if(tile != NULL && tdata.get_terrain_info(terrain).is_combined() && !terrain_info.minimap_image_overlay().empty()) {
 							std::string overlay_file =
 									"terrain/" + terrain_info.minimap_image_overlay() + ".png";
 							surface overlay = get_image(overlay_file,image::HEXED);

src/terrain_builder.cpp

@@ -345,19 +345,25 @@ void terrain_builder::rebuild_terrain(const map_location &loc)
 		btile.images_background.clear();
 		const std::string filename =
 			map().get_terrain_info(loc).minimap_image();
-		animated<image::locator> img_loc;
-		img_loc.add_frame(100,image::locator("terrain/" + filename + ".png"));
-		img_loc.start_animation(0, true);
-		btile.images_background.push_back(img_loc);
+
+		if(!filename.empty()) {
+			animated<image::locator> img_loc;
+			img_loc.add_frame(100,image::locator("terrain/" + filename + ".png"));
+			img_loc.start_animation(0, true);
+			btile.images_background.push_back(img_loc);
+		}
 
 		//Combine base and overlay image if necessary
 		if(map().get_terrain_info(loc).is_combined()) {
 			const std::string filename_ovl =
 				map().get_terrain_info(loc).minimap_image_overlay();
-			animated<image::locator> img_loc_ovl;
-			img_loc_ovl.add_frame(100,image::locator("terrain/" + filename_ovl + ".png"));
-			img_loc_ovl.start_animation(0, true);
-			btile.images_background.push_back(img_loc_ovl);
+
+			if(!filename_ovl.empty()) {
+				animated<image::locator> img_loc_ovl;
+				img_loc_ovl.add_frame(100,image::locator("terrain/" + filename_ovl + ".png"));
+				img_loc_ovl.start_animation(0, true);
+				btile.images_background.push_back(img_loc_ovl);
+			}
 		}
 	}
 }