Do not error on empty shader, just treat it as invalid by default. Fixes #15998.

drivers/gles2/rasterizer_storage_gles2.cpp

@@ -1125,6 +1125,10 @@ void RasterizerStorageGLES2::_update_shader(Shader *p_shader) const {
 
 	p_shader->uniforms.clear();
 
+	if (p_shader->code == String()) {
+		return; //just invalid, but no error
+	}
+
 	ShaderCompilerGLES2::GeneratedCode gen_code;
 	ShaderCompilerGLES2::IdentifierActions *actions = NULL;
 

drivers/gles3/rasterizer_storage_gles3.cpp

@@ -1903,6 +1903,10 @@ void RasterizerStorageGLES3::_update_shader(Shader *p_shader) const {
 
 	p_shader->uniforms.clear();
 
+	if (p_shader->code == String()) {
+		return; //just invalid, but no error
+	}
+
 	ShaderCompilerGLES3::GeneratedCode gen_code;
 	ShaderCompilerGLES3::IdentifierActions *actions = NULL;