build: silence stupid gcc warning on STREQ_NULLABLE

build: silence stupid gcc warning on STREQ_NULLABLE

Our existing STRNEQ_NULLABLE() triggered a warning in gcc 4.7 when
used with a literal NULL argument:

qemumonitorjsontest.c: In function 'testQemuMonitorJSONGetMachines':
qemumonitorjsontest.c:289:5: error: null argument where non-null required (argument 1) [-Werror=nonnull]

even though the strcmp is provably dead when a null argument is
present.  Squelch the warning by refactoring things so that gcc
never sees strcmp() called with NULL arguments (we still compare
NULL as not equal to "", this rewrite merely aids gcc).

Next, gcc has a valid warning about a literal NULLSTR(NULL):

qemumonitorjsontest.c:289:5: error: invalid application of 'sizeof' to a void type [-Werror=pointer-arith]

Of course, you'd never write NULLSTR(NULL) directly, but it is
handy to use through macros.  But the entire part about verify_true()
is unnecessary - gcc already warns about type mismatch with ?:,
without needing to make it more complex.

* src/internal.h (STREQ_NULLABLE, STRNEQ_NULLABLE): Avoid gcc 4.7
stupidity.
(NULLSTR): Simplify, to allow passing compile-time constants.