CompatibleInterface: Test debugging of not-set property.

Tests/RunCMake/CompatibleInterface/DebugProperties-stderr.txt

@@ -66,6 +66,12 @@ CMake Debug Log:
 
    \* Target "CompatibleInterface" has property content "prop3"
 +
+CMake Debug Log:
+  String compatibility of property "STRING_PROP4" for target
+  "CompatibleInterface" \(result: "\(unset\)"\):
+
+   \* Target "CompatibleInterface" property not set.
++
 CMake Debug Log:
   Numeric minimum compatibility of property "NUMBER_MIN_PROP1" for target
   "CompatibleInterface" \(result: "50"\):

Tests/RunCMake/CompatibleInterface/DebugProperties.cmake

@@ -22,6 +22,7 @@ set_property(TARGET iface1 APPEND PROPERTY
     STRING_PROP1
     STRING_PROP2
     STRING_PROP3
+    STRING_PROP4 # Not set.
 )
 set_property(TARGET iface1 APPEND PROPERTY
   COMPATIBLE_INTERFACE_NUMBER_MIN
@@ -36,7 +37,7 @@ set_property(TARGET iface1 APPEND PROPERTY
 
 set(CMAKE_DEBUG_TARGET_PROPERTIES
   BOOL_PROP1 BOOL_PROP2 BOOL_PROP3 BOOL_PROP4 BOOL_PROP5 BOOL_PROP6 BOOL_PROP7
-  STRING_PROP1 STRING_PROP2 STRING_PROP3
+  STRING_PROP1 STRING_PROP2 STRING_PROP3 STRING_PROP4
   NUMBER_MIN_PROP1 NUMBER_MIN_PROP2
   NUMBER_MAX_PROP1 NUMBER_MAX_PROP2
 )