Userland: Fail Core::find_executable_in_path on empty inputs

Before this patch, `which ""` or `type ""` would say that the empty
string is `/usr/local/bin/`.

Convert callers to consistently call is_empty() on the returned string
while we're at it, to support eventually removing the is_null() String
state in the future.

Userland/Libraries/LibCore/DirIterator.cpp

@@ -97,6 +97,9 @@ String DirIterator::next_full_path()
 
 String find_executable_in_path(String filename)
 {
+    if (filename.is_empty())
+        return {};
+
     if (filename.starts_with('/')) {
         if (access(filename.characters(), X_OK) == 0)
             return filename;