use .null? to test NULL pointer returns

we were using `.nil?` in a couple of cases

see #419

thanks dloebl

Author: jcupitt

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## master
 
+* improve NULL pointer handling [dloebl]
+
 ## Version 2.2.4 (2025-06-05)
 
 * fix write to target test with libvips 8.17 [jcupitt]

lib/vips/image.rb

@@ -534,7 +534,6 @@ def self.new_from_array array, scale = 1, offset = 0
       end
 
       image = Vips::Image.matrix_from_array width, height, array
-      raise Vips::Error if image.nil?
 
       image.mutate do |mutable|
         # be careful to set them as double
@@ -700,7 +699,7 @@ def write_to_target target, format_string, **opts
     def write_to_memory
       len = Vips::SizeStruct.new
       ptr = Vips.vips_image_write_to_memory self, len
-      raise Vips::Error if ptr.nil?
+      raise Vips::Error if ptr.null?
 
       # wrap up as an autopointer
       ptr = FFI::AutoPointer.new(ptr, GLib::G_FREE)

lib/vips/interpolate.rb

@@ -50,7 +50,7 @@ class ManagedStruct < Vips::Object::ManagedStruct
     def initialize name
       name = name.to_s if name.is_a? Symbol
       pointer = Vips.vips_interpolate_new name
-      raise Vips::Error if pointer.nil?
+      raise Vips::Error if pointer.null?
 
       super(pointer)
     end

spec/image_spec.rb

@@ -76,6 +76,11 @@
     expect(x.avg).to eq(128)
   end
 
+  it "will raise VipsError for invalid image files" do
+    img = Vips::Image.new_from_file simg("coffee.gif")
+    expect { img.write_to_memory }.to raise_exception(Vips::Error)
+  end
+
   it "throws an error when trying to load an image from memory with unknown size" do
     data = FFI::Pointer.new(1)
     expect { Vips::Image.new_from_memory(data, 16, 16, 1, :uchar) }.to raise_error(Vips::Error)

spec/samples/coffee.gif

@@ -193,5 +193,10 @@
 
       expect { black.crop(10, 10, 1, 1) }.to raise_exception(Vips::Error)
     end
+
+    it "can throw errors for bad interpolations" do
+      expect { Vips::Interpolate.new "banana" }.to raise_exception(Vips::Error)
+    end
+
   end
 end