mesa: avoid NULL ptr in prog parameter name

mesa: avoid NULL ptr in prog parameter name

Context: _mesa_add_parameter is sometimes[0] called with a
NULL name as a mean of an unnamed parameter.

Allowing NULL pointer as a name means that it must be NULL checked
each access. So far it isn't always[1] true.

Parameter name is only used for debug purpose (printf) and
to lookup the index/location of the program by the application.

Conclusion, there is no valid reason to use a NULL pointer instead of
an empty string. So it was decided to use an empty string which avoid all
issues related to NULL pointer

[0]: texture gather offsets glsl opcode and st_init_atifs_prog
[1]: at least shader cache, st_nir_lookup_parameter_index and some printfs

Issue found by piglit 'texturegatheroffsets' tests on Nouveau

v4: new patch based on Nicolai/Timothy/ilia discussion
Signed-off-by: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>