Modified preprocessing of dataframes before customload

src/manifestIO.jl

@@ -367,15 +367,18 @@ function preprocess_dataframe!(df::DataFrame)
     for col in names(df)
         col_type = eltype(df[!, col])
 
-        # Clamp values if range is defined for the column type
+        # Convert missing values to nothing for SQL NULL compatibility
+        df[!, col] = map(x -> ismissing(x) ? nothing : x, df[!, col])
+
+        # Clamp numeric values if range is defined for the column type
         if haskey(SQL_RANGES, col_type)
             min_val, max_val = SQL_RANGES[col_type]
             df[!, col] = clamp.(df[!, col], min_val, max_val)
         end
 
-        # Replace NaN and Inf with nothing for floating-point columns
+        # Replace NaN and Inf with nothing in numeric columns
         if col_type <: AbstractFloat
             df[!, col] = map(x -> isfinite(x) ? x : nothing, df[!, col])
         end
     end
-end
\ No newline at end of file
+end