package_ipk.bbclass: Fix SRC_URI whitespace handling

package_ipk.bbclass: Fix SRC_URI whitespace handling

The SRC_URI may contain whitespace, but be otherwise empty.  This can happen
in the case:

MYSRC = ""
MYSRC_arm = "file://myarm.patch"
SRC_URI += "${MYSRC}"

Unless we strip blank spaces, to determine if it is empty, we can end up
generating a "Source: " line which in invalid.  This leads to the error:

  invalid Source: field is speified in the generated CONTROL file

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>