Shader that ignores only Directional Light?

Anyone? Please, just give me a few ideas how to perform that. Let's say we've got standart Bumped Diffuse shader example:

 Shader "Bumped Diffuse" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
     _MainTex ("Base (RGB)", 2D) = "white" {}
     _BumpMap ("Normalmap", 2D) = "bump" {}
 }
 
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 300
 
 CGPROGRAM
 #pragma surface surf Lambert
 
 sampler2D _MainTex;
 sampler2D _BumpMap;
 fixed4 _Color;
 
 struct Input {
     float2 uv_MainTex;
     float2 uv_BumpMap;
 };
 
 void surf (Input IN, inout SurfaceOutput o) {
     fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
     o.Albedo = c.rgb;
     o.Alpha = c.a;
     o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
 }
 ENDCG  
 }
 
 FallBack "Diffuse"
 }
 

  Shader "Bumped Diffuse" {
  Properties {
      _Color ("Main Color", Color) = (1,1,1,1)
      _MainTex ("Base (RGB)", 2D) = "white" {}
      _BumpMap ("Normalmap", 2D) = "bump" {}
  }
  
  SubShader {
      Tags { "RenderType"="Opaque" }
      LOD 300
  
  CGPROGRAM
  #pragma surface surf LambertCustom
  
  sampler2D _MainTex;
  sampler2D _BumpMap;
  fixed4 _Color;
  
  struct Input {
      float2 uv_MainTex;
      float2 uv_BumpMap;
  };
 
 half4 LightingLambertCustom (SurfaceOutput s, half3 lightDir, half atten) {
     s.Normal = normalize (s.Normal);
     
     half diff;
     if (_WorldSpaceLightPos0.w != 0.0)
         diff = max (0, dot (s.Normal, normalize (lightDir))) * _LightColor0;
     else
         diff = 1.0;
     
     half4 c;
     c.rgb = (s.Albedo * diff) * atten;
     c.a = s.Alpha;
     return c;
 }
  
  void surf (Input IN, inout SurfaceOutput o) {
      fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
      o.Albedo = c.rgb;
      o.Alpha = c.a;
      o.Normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));
  }
  ENDCG  
  }
  
  FallBack "Diffuse"
  }

Something like that should work. Haven't tested, but the w swizzle mask of the _WorldSpaceLightPos variable returns whether the light hitting the object is directional (0.0) or other.