⚡️ Check for empty string using empty string instead of strlen

[turanct]

Using strlen is a tad bit slower on short strings, but it's slower on bigger strings. This commit just compares against empty string instead of counting the length of the string.

Before this change:

Subjects: 3, Assertions: 0, Failures: 0, Errors: 0
suite: 134628b3198fb68334ea8ae03696f4b08d6c05f4, date: 2021-03-15, stime: 20:43:33
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+
| benchmark | subject              | set | revs | its | mem_peak   | best        | mean        | mode        | worst       | stdev     | rstdev | diff      |
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+
| JSONBench | bench_json_encode    | 0   | 5    | 3   | 1,708,160b | 5.200μs     | 5.200μs     | 5.200μs     | 5.200μs     | 0.000μs   | 0.00%  | 1.00x     |
| JSONBench | bench_Parsica_JSON   | 0   | 5    | 3   | 2,194,840b | 6,133.400μs | 6,244.067μs | 6,165.525μs | 6,437.400μs | 137.184μs | 2.20%  | 1,200.78x |
| JSONBench | bench_basemax_jpophp | 0   | 5    | 3   | 1,878,184b | 543.200μs   | 547.800μs   | 544.752μs   | 555.400μs   | 5.414μs   | 0.99%  | 105.35x   |
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+

After this change:

Subjects: 3, Assertions: 0, Failures: 0, Errors: 0
suite: 134628bc5efa46d7260730095c05fc7e10db0222, date: 2021-03-15, stime: 20:43:43
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+
| benchmark | subject              | set | revs | its | mem_peak   | best        | mean        | mode        | worst       | stdev     | rstdev | diff      |
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+
| JSONBench | bench_json_encode    | 0   | 5    | 3   | 1,708,160b | 5.200μs     | 5.333μs     | 5.388μs     | 5.400μs     | 0.094μs   | 1.77%  | 1.00x     |
| JSONBench | bench_Parsica_JSON   | 0   | 5    | 3   | 2,194,552b | 5,426.200μs | 5,959.467μs | 5,700.567μs | 6,678.000μs | 527.569μs | 8.85%  | 1,117.40x |
| JSONBench | bench_basemax_jpophp | 0   | 5    | 3   | 1,878,184b | 531.600μs   | 560.400μs   | 570.972μs   | 581.400μs   | 21.066μs  | 3.76%  | 105.08x   |
+-----------+----------------------+-----+------+-----+------------+-------------+-------------+-------------+-------------+-----------+--------+-----------+

looks like this is is a 10% speed improvement for the json benchmark