Support passing null to b.dependency()

Both null literals and optionals are supported.

test/standalone/dependency_options/build.zig

@@ -12,6 +12,29 @@ pub fn build(b: *std.Build) !void {
     if (!none_specified_mod.resolved_target.?.query.eql(b.graph.host.query)) return error.TestFailed;
     if (none_specified_mod.optimize.? != .Debug) return error.TestFailed;
 
+    // Passing null is the same as not specifying the option,
+    // so this should resolve to the same cached dependency instance.
+    const null_specified = b.dependency("other", .{
+        // Null literals
+        .target = null,
+        .optimize = null,
+        .bool = null,
+
+        // Optionals
+        .int = @as(?i64, null),
+        .float = @as(?f64, null),
+
+        // Optionals of the wrong type
+        .string = @as(?usize, null),
+        .@"enum" = @as(?bool, null),
+
+        // Non-defined option names
+        .this_option_does_not_exist = null,
+        .neither_does_this_one = @as(?[]const u8, null),
+    });
+
+    if (null_specified != none_specified) return error.TestFailed;
+
     const all_specified = b.dependency("other", .{
         .target = b.resolveTargetQuery(.{ .cpu_arch = .x86_64, .os_tag = .windows, .abi = .gnu }),
         .optimize = @as(std.builtin.OptimizeMode, .ReleaseSafe),
@@ -37,6 +60,27 @@ pub fn build(b: *std.Build) !void {
     if (all_specified_mod.resolved_target.?.result.abi != .gnu) return error.TestFailed;
     if (all_specified_mod.optimize.? != .ReleaseSafe) return error.TestFailed;
 
+    const all_specified_optional = b.dependency("other", .{
+        .target = @as(?std.Build.ResolvedTarget, b.resolveTargetQuery(.{ .cpu_arch = .x86_64, .os_tag = .windows, .abi = .gnu })),
+        .optimize = @as(?std.builtin.OptimizeMode, .ReleaseSafe),
+        .bool = @as(?bool, true),
+        .int = @as(?i64, 123),
+        .float = @as(?f64, 0.5),
+        .string = @as(?[]const u8, "abc"),
+        .string_list = @as(?[]const []const u8, &.{ "a", "b", "c" }),
+        .lazy_path = @as(?std.Build.LazyPath, .{ .cwd_relative = "abc.txt" }),
+        .lazy_path_list = @as(?[]const std.Build.LazyPath, &.{
+            .{ .cwd_relative = "a.txt" },
+            .{ .cwd_relative = "b.txt" },
+            .{ .cwd_relative = "c.txt" },
+        }),
+        .@"enum" = @as(?Enum, .alfa),
+        //.enum_list = @as(?[]const Enum, &.{ .alfa, .bravo, .charlie }),
+        //.build_id = @as(?std.zig.BuildId, .uuid),
+    });
+
+    if (all_specified_optional != all_specified) return error.TestFailed;
+
     // Most supported option types are serialized to a string representation,
     // so alternative representations of the same option value should resolve
     // to the same cached dependency instance.
@@ -59,5 +103,5 @@ pub fn build(b: *std.Build) !void {
         //.build_id = @as(std.zig.BuildId, .uuid),
     });
 
-    if (all_specified != all_specified_alt) return error.TestFailed;
+    if (all_specified_alt != all_specified) return error.TestFailed;
 }