[library] Fix ub-sanitizer error 'applying non-zero offset to null pointer'

```
   #0 0x23d0d55b in GetSchemePrefixSize(TBasicStringBuf<char, std::__y1::char_traits<char> >) /home/vmordovin/src/arcadia/fuse/arcadia/library/cpp/string_utils/url/url.cpp:111:11
    #1 0x23d0ce99 in GetSchemeHostAndPort(TBasicStringBuf<char, std::__y1::char_traits<char> >, bool, bool) /home/vmordovin/src/arcadia/fuse/arcadia/library/cpp/string_utils/url/url.cpp:173:31
    #2 0x23d0da8b in NUrl::SplitUrlToHostAndPath(TBasicStringBuf<char, std::__y1::char_traits<char> >) /home/vmordovin/src/arcadia/fuse/arcadia/library/cpp/string_utils/url/url.cpp:68:27
    #3 0x23d0da8b in SplitUrlToHostAndPath(TBasicStringBuf<char, std::__y1::char_traits<char> >, TBasicStringBuf<char, std::__y1::char_traits<char> >&, TBasicStringBuf<char, std::__y1::char_traits<char> >&) /hom
e/vmordovin/src/arcadia/fuse/arcadia/library/cpp/string_utils/url/url.cpp:201:31
    #4 0x2c0d8715 in NReportSnippets::SplitGreenUrl(NSc::TValue&, TBasicStringBuf<char, std::__y1::char_traits<char> >, TBasicStringBuf<char, std::__y1::char_traits<char> >) /home/vmordovin/src/arcadia/fuse/arca
dia/search/web/util/report_snippets/report_snippets.cpp:50:5
    #5 0x2363d63b in (anonymous namespace)::TReportSnippetGenericContext::ProcessGrouping(TMetaGrouping&, TGroupingIndex const&, IMetaRearrangeContext::TRearrangeParams const&) /home/vmordovin/src/arcadia/fuse/a
rcadia/search/web/rearrange/rep_snip_generic/generic.cpp:129:21
...
```

Author: vmordovin

library/cpp/string_utils/url/url.cpp

@@ -98,6 +98,10 @@ TWtringBuf CutHttpPrefix(const TWtringBuf url, bool ignorehttps) noexcept {
 }
 
 size_t GetSchemePrefixSize(const TStringBuf url) noexcept {
+    if (url.empty()) {
+        return 0;
+    }
+
     struct TDelim: public str_spn {
         inline TDelim()
             : str_spn("!-/:-@[-`{|}", true)
@@ -433,4 +437,3 @@ bool DoesUrlPathStartWithToken(TStringBuf url, const TStringBuf& token) noexcept
     const bool prefixIsToken = slashAfterPrefix || qMarkAfterPrefix || nothingAfterPrefix;
     return prefixIsToken;
 }
-