fix: various method ambiguities

Author: MilesCranmer

test/test_multi_expression.jl

@@ -87,10 +87,15 @@
         )::Expression{T,N}
         return fused_expression.tree
     end
-    function DE.get_operators(ex::MultiScalarExpression, operators=nothing)
+    function DE.get_operators(
+        ex::MultiScalarExpression, operators::Union{AbstractOperatorEnum,Nothing}=nothing
+    )
         return operators === nothing ? ex.metadata.operators : operators
     end
-    function DE.get_variable_names(ex::MultiScalarExpression, variable_names=nothing)
+    function DE.get_variable_names(
+        ex::MultiScalarExpression,
+        variable_names::Union{Nothing,AbstractVector{<:AbstractString}}=nothing,
+    )
         return variable_names === nothing ? ex.metadata.variable_names : variable_names
     end
     function Base.copy(ex::MultiScalarExpression)