Backends: OpenGL3: call glGetString(GL_VERSION) even in GS ES 2.0 path. (#8197)

Apparently as per specs works. I reckon the best way to confirm it is to try.

backends/imgui_impl_opengl3.cpp

@@ -297,15 +297,13 @@ bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
     io.BackendRendererName = "imgui_impl_opengl3";
 
     // Query for GL version (e.g. 320 for GL 3.2)
+    const char* gl_version_str = (const char*)glGetString(GL_VERSION);
 #if defined(IMGUI_IMPL_OPENGL_ES2)
     // GLES 2
-    const char* gl_version_str = "";
-    IM_UNUSED(gl_version_str); // For IMGUI_IMPL_OPENGL_DEBUG block below.
     bd->GlVersion = 200;
     bd->GlProfileIsES2 = true;
 #else
     // Desktop or GLES 3
-    const char* gl_version_str = (const char*)glGetString(GL_VERSION);
     GLint major = 0;
     GLint minor = 0;
     glGetIntegerv(GL_MAJOR_VERSION, &major);