build/i18n-scan.pl: avoid a possible endless loop

Issuing './build/i18n-scan.pl libs/core/' leads to this
endless loop with some perl implementations:

<...>
Use of uninitialized value in concatenation (.) or string at../build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
Use of uninitialized value in concatenation (.) or string at ./build/i18n-scan.pl line 69.
substr outside of string at ./build/i18n-scan.pl line 69.
<...>

Experienced with this perl version:
"This is perl, v5.8.8 built for i386-linux"

Fix it by undefining the 'sub' variable if it is an
empty string.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>