Send an empty dictionary when calling /join to be spec compliant.

[pixlwave]

The SDK isn't sending spec-complaint joins, and Synapse is about to stop handling these: matrix-org/synapse#13388.

Fixes element-hq/element-ios#6481.

[ismailgulek]

LGTM!

[codecov-commenter]

Codecov Report

Merging #1539 (fb326d8) into develop (93670ea) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1539   +/-   ##
========================================
  Coverage    51.19%   51.20%           
========================================
  Files          514      514           
  Lines        83952    83956    +4     
  Branches     37409    37416    +7     
========================================
+ Hits         42983    42989    +6     
+ Misses       39866    39865    -1     
+ Partials      1103     1102    -1     

Impacted Files	Coverage Δ
MatrixSDK/MXRestClient.m	45.65% <100.00%> (-0.02%)	⬇️
...Crypto/CrossSigning/JSONModels/MXCrossSigningKey.m	92.39% <0.00%> (-1.09%)	⬇️
MatrixSDK/Data/Store/MXMemoryStore/MXMemoryStore.m	65.80% <0.00%> (-0.87%)	⬇️
MatrixSDK/Utils/MXHTTPClient.m	51.10% <0.00%> (-0.51%)	⬇️
...Data/Store/MXRealmCryptoStore/MXRealmCryptoStore.m	72.21% <0.00%> (-0.15%)	⬇️
MatrixSDK/MXSession.m	50.68% <0.00%> (-0.13%)	⬇️
...SDK/Crypto/Verification/MXKeyVerificationManager.m	72.36% <0.00%> (+0.21%)	⬆️
MatrixSDK/Crypto/MXCrypto.m	71.95% <0.00%> (+0.49%)	⬆️
...rixSDK/Aggregations/MXAggregatedReactionsUpdater.m	86.39% <0.00%> (+0.64%)	⬆️
... and 1 more

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