Handle "plugin not found" cases more gracefully

OpenVINO can be configured with a `plugins.xml` file that points to
libraries for running inferences on other devices. Previously, if this
file was not provided and `openvino-finder` could not find it, the
bindings crate would fail with an error. But this is not always
necessary: if the plugin libraries are located in the same directory as
the OpenVINO library, then a `plugins.xml` file is not needed. This is
the case for APT-installed versions of the package, e.g. In these cases,
we can pass an empty string to inform OpenVINO to look in the same
directory as the main library ([example]).

[example]: https://github.com/openvinotoolkit/openvino/blob/05a24b17769cb634f16343d551e25f542f0855b1/docs/snippets/ie_common.c#L14

Author: abrown

crates/openvino/src/core.rs

@@ -29,19 +29,19 @@ impl Core {
     pub fn new(xml_config_file: Option<&str>) -> std::result::Result<Core, SetupError> {
         openvino_sys::library::load().map_err(LoadingError::SystemFailure)?;
 
-        let file = match xml_config_file {
-            None => openvino_finder::find_plugins_xml()
-                .ok_or(LoadingError::CannotFindPluginPath)?
+        let file = if let Some(file) = xml_config_file {
+            cstr!(file.to_string())
+        } else if let Some(file) = openvino_finder::find_plugins_xml() {
+            cstr!(file
                 .to_str()
                 .ok_or(LoadingError::CannotStringifyPath)?
-                .to_string(),
-            Some(f) => f.to_string(),
+                .to_string())
+        } else {
+            cstr!("".to_string())
         };
+
         let mut instance = std::ptr::null_mut();
-        try_unsafe!(ie_core_create(
-            cstr!(file),
-            std::ptr::addr_of_mut!(instance)
-        ))?;
+        try_unsafe!(ie_core_create(file, std::ptr::addr_of_mut!(instance)))?;
         Ok(Core { instance })
     }