fix "Trying to access array offset on value of type bool" in 7.4

[remicollet]

Without:

1) Sabre\HTTP\Auth\DigestTest::testInvalidDigest2
Trying to access array offset on value of type bool

/work/GIT/sabre-http/lib/Auth/Digest.php:124
/work/GIT/sabre-http/lib/Auth/Digest.php:94
/work/GIT/sabre-http/tests/HTTP/Auth/DigestTest.php:99

Also see sabre-io/dav#1187

[codecov]

Codecov Report

Merging #121 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #121      +/-   ##
============================================
+ Coverage     93.51%   93.52%   +0.01%     
- Complexity      250      251       +1     
============================================
  Files            15       15              
  Lines           817      819       +2     
============================================
+ Hits            764      766       +2     
  Misses           53       53

Impacted Files	Coverage Δ	Complexity Δ
lib/Auth/Digest.php	96.42% <100%> (+0.13%)	20 <0> (+1)	⬆️

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[staabm]

thx remi