Trade: AbstractImporter::importerState() expects a file to be opened.

The restriction was there always, but was undocumented and untested.

src/Magnum/Trade/AbstractImporter.h

@@ -280,8 +280,8 @@ checked by the implementation:
 -   The @ref doSetFileCallback() function is called only if
     @ref ImporterFeature::FileCallback is supported and there is no file
     opened.
--   All `do*()` implementations working on an opened file are called only if
+-   All `do*()` implementations working on an opened file as well as
-    there is any file opened.
+    @ref doImporterState() are called only if there is any file opened.
 -   All `do*()` implementations taking data ID as parameter are called only if
     the ID is from valid range.
 -   For @ref doMesh() and `doImage*()` and @p level parameter being nonzero,
@@ -1280,6 +1280,7 @@ class MAGNUM_TRADE_EXPORT AbstractImporter: public PluginManager::AbstractManagi
          * for currently opened document through this function. See
          * documentation of a particular plugin for more information about
          * returned type and contents. Returns @cpp nullptr @ce by default.
+         * Expects that a file is opened.
          * @see @ref AbstractMaterialData::importerState(),
          *      @ref AnimationData::importerState(), @ref CameraData::importerState(),
          *      @ref ImageData::importerState(), @ref LightData::importerState(),

src/Magnum/Trade/Test/AbstractImporterTest.cpp

@@ -1297,6 +1297,8 @@ void AbstractImporterTest::thingNoFile() {
     importer.image3D(42);
     importer.image3D("foo");
 
+    importer.importerState();
+
     CORRADE_COMPARE(out.str(),
         "Trade::AbstractImporter::defaultScene(): no file opened\n"
         "Trade::AbstractImporter::scene(): no file opened\n"
@@ -1325,7 +1327,9 @@ void AbstractImporterTest::thingNoFile() {
         "Trade::AbstractImporter::image2D(): no file opened\n"
         "Trade::AbstractImporter::image2D(): no file opened\n"
         "Trade::AbstractImporter::image3D(): no file opened\n"
-        "Trade::AbstractImporter::image3D(): no file opened\n");
+        "Trade::AbstractImporter::image3D(): no file opened\n"
+
+        "Trade::AbstractImporter::importerState(): no file opened\n");
 }
 
 void AbstractImporterTest::defaultScene() {