[libxml2] Fix handling of XSD with empty namespace

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 1c7d91abe404333cb29f547ae7e844467befc53a
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Feb 3 23:31:19 2022 +0100

    Fix handling of XSD with empty namespace
    
    An empty namespace means no default namespace.
    
    Fixes #303.

 result/schemas/issue303_0_0 |  1 +
 test/schemas/issue303_0.xml |  1 +
 test/schemas/issue303_0.xsd | 19 +++++++++++++++++++
 xmlschemas.c                |  2 +-
 4 files changed, 22 insertions(+), 1 deletion(-)
---
diff --git a/result/schemas/issue303_0_0 b/result/schemas/issue303_0_0
new file mode 100644
index 00000000..6deaf66f
--- /dev/null
+++ b/result/schemas/issue303_0_0
@@ -0,0 +1 @@
+./test/schemas/issue303_0.xml validates
diff --git a/test/schemas/issue303_0.xml b/test/schemas/issue303_0.xml
new file mode 100644
index 00000000..53bddb28
--- /dev/null
+++ b/test/schemas/issue303_0.xml
@@ -0,0 +1 @@
+<outer> <inner><a>1</a><b>2</b></inner> <c>3</c> </outer>
diff --git a/test/schemas/issue303_0.xsd b/test/schemas/issue303_0.xsd
new file mode 100644
index 00000000..d85efb86
--- /dev/null
+++ b/test/schemas/issue303_0.xsd
@@ -0,0 +1,19 @@
+<?xml version="1.0"?>
+<xs:schema id="foo" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema";>
+  <xs:element name="inner">
+    <xs:complexType>
+      <xs:sequence>
+        <xs:element name="a" type="xs:double" minOccurs="1" />
+        <xs:element name="b" type="xs:double" minOccurs="1" />
+      </xs:sequence>
+    </xs:complexType>
+  </xs:element>
+  <xs:element name="outer">
+    <xs:complexType>
+      <xs:sequence>
+        <xs:element ref="inner" minOccurs="0" maxOccurs="unbounded" />
+        <xs:element name="c" type="xs:double" minOccurs="1" />
+      </xs:sequence>
+    </xs:complexType>
+  </xs:element>
+</xs:schema>
diff --git a/xmlschemas.c b/xmlschemas.c
index a8019bea..64fe146a 100644
--- a/xmlschemas.c
+++ b/xmlschemas.c
@@ -5899,7 +5899,7 @@ xmlSchemaPValAttrNodeQNameValue(xmlSchemaParserCtxtPtr ctxt,
 
     if (!strchr((char *) value, ':')) {
        ns = xmlSearchNs(attr->doc, attr->parent, NULL);
-       if (ns)
+       if (ns && ns->href && ns->href[0])
            *uri = xmlDictLookup(ctxt->dict, ns->href, -1);
        else if (schema->flags & XML_SCHEMAS_INCLUDING_CONVERT_NS) {
            /* TODO: move XML_SCHEMAS_INCLUDING_CONVERT_NS to the