Support blank keyword (#38)

Fixes #37 

```release-note
None
```

Author: JohnNiang

src/main/java/run/halo/search/widget/SearchView.java

@@ -6,8 +6,10 @@
 import org.springframework.web.reactive.function.server.*;
 import reactor.core.publisher.Mono;
 import run.halo.app.search.SearchOption;
+import run.halo.app.search.SearchResult;
 import run.halo.app.search.SearchService;
 
+import java.util.List;
 import java.util.Map;
 
 @Component
@@ -27,19 +29,29 @@ RouterFunction<ServerResponse> searchView() {
     }
 
     private Mono<ServerResponse> performSearch(ServerRequest serverRequest) {
-        var keyword = serverRequest.queryParam("keyword").orElse("");
-        var limit = serverRequest.queryParam("limit")
+        int limit = serverRequest.queryParam("limit")
                 .map(Integer::parseInt)
                 .orElse(10);
-        var includeTypes = serverRequest.queryParams().get("includeTypes");
-        var option = new SearchOption();
-        option.setKeyword(keyword);
-        option.setLimit(limit);
-        option.setFilterExposed(true);
-        option.setFilterPublished(true);
-        option.setFilterRecycled(false);
-        option.setIncludeTypes(includeTypes);
-        var result = searchService.search(option);
+        var keyword = serverRequest.queryParam("keyword").orElse("");
+        Mono<SearchResult> result;
+        if (keyword.isBlank()) {
+            var searchResult = new SearchResult();
+            searchResult.setTotal(0L);
+            searchResult.setLimit(limit);
+            searchResult.setProcessingTimeMillis(0);
+            searchResult.setHits(List.of());
+            result = Mono.just(searchResult);
+        } else {
+            var includeTypes = serverRequest.queryParams().get("includeTypes");
+            var option = new SearchOption();
+            option.setKeyword(keyword);
+            option.setLimit(limit);
+            option.setFilterExposed(true);
+            option.setFilterPublished(true);
+            option.setFilterRecycled(false);
+            option.setIncludeTypes(includeTypes);
+            result = searchService.search(option);
+        }
         return ServerResponse.ok().render("search", Map.of(
                 "searchResult", result
         ));