Embind: Consistently represent empty interface references as JS null

The existing code had two issues:  For one, e.g. calling a function that
returned a null interface reference mapped that to JS null, while querying an
object for a non-supported interface via `new css.uno.X...(y)` mapped that to
an "empty" JS object instead.  (So checking for a non-null reference needed to
be done as either `x !=== null` or as `x.is()`, depending on context.)  And for
another, while calling $is() on a non-"empty" object worked fine, it failed on
such an "empty" object (as the Embind internals of that object lacked the $$
property).

So change the querying mechanism from the `new css.uno.X...(y)` constructor to a
`css.uno.X....query(y)` function call syntax, which now returns JS null when
querying for a non-supported interface.  (And drop the broken $is method.)

Change-Id: I8fa488ab2892423f2a461d9b72db47e1d4df3b8d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/166255
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

1 files changed, 2 insertions, 0 deletions

diff --git a/offapi/org/libreoffice/embindtest/XTest.idl b/offapi/org/libreoffice/embindtest/XTest.idl
index 8817e94669fc..3dc1773c120a 100644
--- a/offapi/org/libreoffice/embindtest/XTest.idl
+++ b/offapi/org/libreoffice/embindtest/XTest.idl
@@ -112,6 +112,8 @@ interface XTest {
     boolean isSequenceEnum([in] sequence<Enum> value);
     sequence<Struct> getSequenceStruct();
     boolean isSequenceStruct([in] sequence<Struct> value);
+    XTest getNull();
+    boolean isNull([in] XTest value);
     void throwRuntimeException();
     void passJob([in] com::sun::star::task::XJob object);
     void passJobExecutor([in] com::sun::star::task::XJobExecutor object);