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

@@ -585,8 +585,10 @@ void ShaderSource::rebuildShaders()
 	// Recreate shaders
 	for(u32 i=0; i<m_shaderinfo_cache.size(); i++){
 		ShaderInfo *info = &m_shaderinfo_cache[i];
-		*info = generate_shader(info->name, m_device,
-				m_shader_callback, &m_sourcecache);
+		if(info->name != ""){
+			*info = generate_shader(info->name, m_device,
+					m_shader_callback, &m_sourcecache);
+		}
 	}
 }