Combining textures in custom shader

Hi,
Is it possible to define an "empty transparent texture" as a default, or somehow detect if no texture was assigned, in a shader?
I have a custom shader where several layers are combined - everything works OK if I assign all the slots. But if I left secondary texture slot empty, it looks like a white color is passed instead.

Code (CSharp):

1.  
2. _Color ("Main Color", Color) = (1,1,1,1)
3. _SkinTex ("Skin Texture (RGBA)", 2D) = "white" {}
4. _TattooTex ("Tattoo Texture (RGBA)", 2D) = "" {}
5. _TattooAlpha ("Tattoo Alpha", Range (0, 1)) = 0
6. _ScarTex ("Scar Texture (RGBA)", 2D) = "" {}
7. _ScarAlpha ("Scar Alpha", Range (0, 1)) = 0
8.  

...

Code (CSharp):

1.  
2. void surf(Input IN, inout SurfaceOutputStandard o)
3.         {
4.             half4 c = tex2D(_SkinTex, IN.uv_SkinTex) * _Color;
5.             half4 tattoo = tex2D(_TattooTex, IN.uv_TattooTex);
6.             half4 scar =   tex2D(_ScarTex, IN.uv_ScarTex);
7.             c.rgb = lerp (c.rgb, tattoo.rgb, tattoo.a * _TattooAlpha);
8.             c.rgb = lerp (c.rgb, scar.rgb, scar.a * _ScarAlpha);
9.             o.Albedo = c.rgb * _Color.rgb;
10.  

Is it possible to ignore a secondary map if it is not assigned?
The only workaround I have found for now is assigning a transparent image as a secondary map if I don't need it.

Thank you.

 

Update:
Resolved by setting all _...Alpha sliders to 0 by default.