mesa: fix empty program log length

mesa: fix empty program log length

In case we have empty log (""), we should return 0. This fixes
Khronos WebGL conformance test 'program-infolog'.

From OpenGL ES 3.1 (and OpenGL 4.5 Core) spec:
   "If pname is INFO_LOG_LENGTH , the length of the info log, including
    a null terminator, is returned. If there is no info log, zero is
    returned."

v2: apply same fix for get_shaderiv and _mesa_GetProgramPipelineiv (Ian)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> (v1)
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97321
Cc: "13.0" <mesa-stable@lists.freedesktop.org>