cargo subprojects: don't assume each target has dependencies

Previously, creating a meson wrap with "method=cargo"
for https://github.com/sunfishcode/is-terminal would fail,
because its Cargo.toml contains a [target] section without
dependencies.

Instead, we now fall back to an empty list in case of no specified dependencies
(like everywhere else in "_convert_manifest")

mesonbuild/cargo/interpreter.py

@@ -292,7 +292,7 @@ def _convert_manifest(raw_manifest: manifest.Manifest, subdir: str, path: str =
         [Benchmark(**_fixup_raw_mappings(b)) for b in raw_manifest.get('bench', {})],
         [Example(**_fixup_raw_mappings(b)) for b in raw_manifest.get('example', {})],
         raw_manifest.get('features', {}),
-        {k: {k2: Dependency.from_raw(v2) for k2, v2 in v['dependencies'].items()}
+        {k: {k2: Dependency.from_raw(v2) for k2, v2 in v.get('dependencies', {}).items()}
          for k, v in raw_manifest.get('target', {}).items()},
         subdir,
         path,