Don't rebuild the dummy shader (less SourceShaderCache warning spam)

Also write something about rebuilding shaders to infostream in
Client::afterContentReceived()

src/client.cpp

@@ -2850,6 +2850,7 @@ void Client::afterContentReceived(IrrlichtDevice *device, gui::IGUIFont* font)
 	m_tsrc->rebuildImagesAndTextures();
 
 	// Rebuild shaders
+	infostream<<"- Rebuilding shaders"<<std::endl;
 	m_shsrc->rebuildShaders();
 
 	// Update node aliases

src/shader.cpp

@@ -585,10 +585,12 @@ void ShaderSource::rebuildShaders()
 	// Recreate shaders
 	for(u32 i=0; i<m_shaderinfo_cache.size(); i++){
 		ShaderInfo *info = &m_shaderinfo_cache[i];
+		if(info->name != ""){
 			*info = generate_shader(info->name, m_device,
 					m_shader_callback, &m_sourcecache);
 		}
 	}
+}
 
 void ShaderSource::onSetConstants(video::IMaterialRendererServices *services,
 		bool is_highlevel, const std::string &name)