AK: Don't return null from String[View]::substring_view()

We expect the result to be usable with the
StringView::substring_view_starting_*_substring() methods.

See https://github.com/SerenityOS/serenity/pull/938

AK/String.cpp

@@ -99,8 +99,6 @@ String String::substring(size_t start, size_t length) const
 
 StringView String::substring_view(size_t start, size_t length) const
 {
-    if (!length)
-        return {};
     ASSERT(m_impl);
     ASSERT(start + length <= m_impl->length());
     // FIXME: This needs some input bounds checking.

AK/StringView.cpp

@@ -105,8 +105,6 @@ bool StringView::ends_with(const StringView& str) const
 
 StringView StringView::substring_view(size_t start, size_t length) const
 {
-    if (!length)
-        return {};
     ASSERT(start + length <= m_length);
     return { m_characters + start, length };
 }