Ignore empty-set environment values for config

[AJIOB]

We face with problem on Linux, when WebDAV branch was incorrectly triggered, because the empty WebDAV endpoint was exported from Bash and incorrectly interpreted by sccache.

Error we faced:

sccache: Starting the server...
sccache: error: Server startup failed: create webdav cache failed: ConfigInvalid (permanent) at , context: \{ service: webdav } => endpoint is empty
  
Stack backtrace:
   0: <unknown>
   1: <unknown>
   2: <unknown>
   3: <unknown>
   4: <unknown>
   5: <unknown>
   6: <unknown>
   7: <unknown>

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.20290% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.39%. Comparing base (7b751fc) to head (f2073eb).

Files with missing lines	Patch %	Lines
src/config.rs	94.20%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2639      +/-   ##
==========================================
+ Coverage   73.37%   73.39%   +0.02%     
==========================================
  Files          68       68              
  Lines       37338    37355      +17     
==========================================
+ Hits        27396    27417      +21     
+ Misses       9942     9938       -4     

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[AJIOB]

Hi @sylvestre ,

Is this PR OK for you?