[MUC] Fix handling of empty room topic lm_message_node_get_child_value() didn't make any difference between a missing node and an empty one. With this patch it will return "" if the node exists but has no value.

SUBDIRS = mcabber doc modules
ACLOCAL_AMFLAGS = -I macros
if INSTALL_HEADERS
pkgconfigdir = $(libdir)/pkgconfig
pkgconfig_DATA = mcabber.pc
endif