Text: allow empty files in AbstractFont.

In particular, MagnumFont can be an empty.

doc/changelog.dox

@@ -122,6 +122,12 @@ See also:
     experience for example when using ANGLE on Windows. See
     @ref Platform-Sdl2Application-usage-gles for more information.
 
+@subsubsection changelog-latest-changes-text Text library
+
+-   For consistency with @ref Trade::AbstractImporter, @ref Text::AbstractFont
+    now doesn't fail when opening empty files --- instead this is handled in
+    the particular plugin implementations
+
 @subsubsection changelog-latest-changes-trade Trade library
 
 -   The @ref Trade::AnyImageImporter "AnyImageImporter",

src/Magnum/Text/AbstractFont.cpp

@@ -63,9 +63,10 @@ AbstractFont::AbstractFont(PluginManager::AbstractManager& manager, const std::s
 bool AbstractFont::openData(const std::vector<std::pair<std::string, Containers::ArrayView<const char>>>& data, const Float size) {
     CORRADE_ASSERT(features() & Feature::OpenData,
         "Text::AbstractFont::openData(): feature not supported", false);
-    CORRADE_ASSERT(!data.empty(),
-        "Text::AbstractFont::openData(): no data passed", false);
 
+    /* We accept empty data here (instead of checking for them and failing so
+       the check doesn't be done on the plugin side) because for some file
+       formats it could be valid (MagnumFont in particular). */
     close();
     const Metrics metrics = doOpenData(data, size);
     _size = metrics.size;