Conditional input of texture into a shader

Sometimes the shader I am creating will be given an image file to use as a texture sampler and sometimes it will not. 

In the shader when the image file is given I need the line

uniform sampler2D refTexture;

and in the main function a line along the lines of

float x = 0.5 * texture2D(refSampler, uv).rgb.x;

and when no image file is given then

float x = 0.5 * position.x

From looking at the code in the samplers in the materialsLibrary I see there are some conditions on the uniforms such as

#if defined(SPOTLIGHT0) || defined(DIRLIGHT0)
varying vec4 vPositionFromLight0;
uniform sampler2D shadowSampler0;
#else
uniform samplerCube shadowSampler0;
#endif

1. Is there a condition I can use for

#if ..................
uniform sampler2D refTexture;
#endif

2. Is there a conditional statement within the main function I can use for

IF texture loaded
    float x = 0.5 * texture2D(refSampler, uv).rgb.x;
ELSE
    float x = 0.5 * position.x
ENDIF