Shaders: instancedNormalMatrix isn't needed if Phong has zero lights.

And add an explicit construction test for this case.

src/Magnum/Shaders/Phong.vert

@@ -242,11 +242,13 @@ layout(location = TRANSFORMATION_MATRIX_ATTRIBUTE_LOCATION)
 #endif
 in highp mat4 instancedTransformationMatrix;
 
+#ifdef HAS_LIGHTS
 #ifdef EXPLICIT_ATTRIB_LOCATION
 layout(location = NORMAL_MATRIX_ATTRIBUTE_LOCATION)
 #endif
 in highp mat3 instancedNormalMatrix;
 #endif
+#endif
 
 #ifdef INSTANCED_TEXTURE_OFFSET
 #ifdef EXPLICIT_ATTRIB_LOCATION

src/Magnum/Shaders/Test/PhongGLTest.cpp

@@ -247,6 +247,7 @@ constexpr struct {
     {"five lights", {}, 5},
     {"zero lights", {}, 0},
     {"instanced transformation", PhongGL::Flag::InstancedTransformation, 3},
+    {"instanced transformation, zero lights", PhongGL::Flag::InstancedTransformation, 0},
     {"instanced specular texture offset", PhongGL::Flag::SpecularTexture|PhongGL::Flag::InstancedTextureOffset, 3},
     {"instanced normal texture offset", PhongGL::Flag::NormalTexture|PhongGL::Flag::InstancedTextureOffset, 3},
     #ifndef MAGNUM_TARGET_GLES2