localed: normalize empty X11 option values to NULL after parsing

x11_read_data() parses an 'Option "XkbVariant" ""' line in
00-keyboard.conf with strv_split_full(..., EXTRACT_UNQUOTE), which turns
the empty quoted value into a non-NULL empty string rather than NULL.
Since 812aa57d2c ("string-util: beef up string_is_safe()") an empty
string is rejected by string_is_safe() unless STRING_ALLOW_EMPTY is
passed, so x11_context_is_safe() now refuses such a context and
x11_context_verify() discards the whole thing. As a result "localectl
status" reports "X11 Layout: (unset)" even though the file names a valid
layout, and compositors reading org.freedesktop.locale1 (e.g. the SDDM
greeter) fall back to the us layout.

Introduce x11_context_normalize(), suggested by @lionheartyu, which
converts empty strings to NULL while freeing the heap allocation —
unlike x11_context_empty_to_null() which only NULLs the pointer without
freeing. Call it at the end of x11_read_data(), before
x11_context_verify(), so empty option values are treated as unset. This
also keeps x11_context_equal() comparisons consistent with the setter
path (method_set_x11_keyboard) and vconsole_read_data(), which both
store NULL for empty values.

Fixes #43007

src/shared/vconsole-util.c

@@ -84,6 +84,25 @@ void x11_context_empty_to_null(X11Context *xc) {
         xc->options = empty_to_null(xc->options);
 }
 
+static char* empty_to_null_and_free(char *p) {
+        return isempty(p) ? mfree(p) : p;
+}
+
+void x11_context_normalize(X11Context *xc) {
+        assert(xc);
+
+        /* Like x11_context_empty_to_null(), but also frees the heap memory
+         * owned by the fields. Use this for contexts whose strings are
+         * heap-allocated (e.g. x11_read_data()), not for borrowed contexts
+         * such as the one built by method_set_x11_keyboard() from a D-Bus
+         * message. */
+
+        xc->layout  = empty_to_null_and_free(xc->layout);
+        xc->model   = empty_to_null_and_free(xc->model);
+        xc->variant = empty_to_null_and_free(xc->variant);
+        xc->options = empty_to_null_and_free(xc->options);
+}
+
 bool x11_context_is_safe(const X11Context *xc) {
         assert(xc);
 

src/shared/vconsole-util.h

@@ -19,6 +19,7 @@ void x11_context_clear(X11Context *xc);
 void x11_context_replace(X11Context *dest, X11Context *src);
 bool x11_context_isempty(const X11Context *xc);
 void x11_context_empty_to_null(X11Context *xc);
+void x11_context_normalize(X11Context *xc);
 bool x11_context_is_safe(const X11Context *xc);
 bool x11_context_equal(const X11Context *a, const X11Context *b);
 int x11_context_copy(X11Context *dest, const X11Context *src);