reg: allow all-zeros as name for fields
reg: allow all-zeros as name for fields

Assume a register-diagram with all-zeros for one of the register fields:
       { reg:[ { bits:  7, name: 0x3b, attr: ['OP-32'] },
	       { bits:  5, name: 'rd' },
	       { bits:  3, name: 0x0, attr: ['ADD.UW'] },
	       { bits:  5, name: 'rs1' },
	       { bits:  5, name: 'rs2' },
	       { bits:  7, name: 0x04, attr: ['ADD.UW'] } ] }

We expect the field with 3 bits to be labeled as 0|0|0.  However, the
current implementation skips printing the name, as it does not
differentiate between an unnamed field (i.e. without any name defined)
or one that has the integer-value 0 as its name.

This adjusts the behaviour by explicitly checking whether name is
undefined to determine whether the name should be printed.

Signed-off-by: Philipp Tomsich <16b450d03fd5fb9ae9cd9b00b81d3f9ed09da059@vrull.eu>

drom/bitfield