Use === for checks against nothing

The compiler likes this a lot more

src/atoms/affine/index.jl

@@ -40,7 +40,7 @@ function curvature(x::IndexAtom)
 end
 
 function evaluate(x::IndexAtom)
-    if x.inds == nothing
+    if x.inds === nothing
         return getindex(evaluate(x.children[1]), x.rows, x.cols)
     else
         return getindex(evaluate(x.children[1]), x.inds)
@@ -52,7 +52,7 @@ function conic_form!(x::IndexAtom, unique_conic_forms::UniqueConicForms=UniqueCo
         m = length(x)
         n = length(x.children[1])
 
-        if x.inds == nothing
+        if x.inds === nothing
             sz = length(x.cols) * length(x.rows)
             J = Array{Int}(undef, sz)
             k = 1

src/variable.jl

@@ -64,7 +64,7 @@ function vexity(x::Variable)
 end
 
 function evaluate(x::Variable)
-    return x.value == nothing ? error("Value of the variable is yet to be calculated") : x.value
+    return x.value === nothing ? error("Value of the variable is yet to be calculated") : x.value
 end
 
 function sign(x::Variable)
@@ -114,7 +114,7 @@ end
 
 # fix variables to hold them at their current value, and free them afterwards
 function fix!(x::Variable)
-    x.value == nothing && error("This variable has no value yet; cannot fix value to nothing!")
+    x.value === nothing && error("This variable has no value yet; cannot fix value to nothing!")
     push!(x.sets, :fixed)
     x.vexity = ConstVexity()
     x